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-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE2 man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcre2demo program. There are separate text files for the pcre2grep and
pcre2test commands.
-----------------------------------------------------------------------------
PCRE2(3) Library Functions Manual PCRE2(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
INTRODUCTION
PCRE2 is the name used for a revised API for the PCRE library, which is
a set of functions, written in C, that implement regular expression
pattern matching using the same syntax and semantics as Perl, with just
a few differences. Some features that appeared in Python and the origi-
nal PCRE before they appeared in Perl are also available using the
Python syntax. There is also some support for one or two .NET and Onig-
uruma syntax items, and there are options for requesting some minor
changes that give better ECMAScript (aka JavaScript) compatibility.
The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or
32-bit code units, which means that up to three separate libraries may
be installed. The original work to extend PCRE to 16-bit and 32-bit
code units was done by Zoltan Herczeg and Christian Persch, respec-
tively. In all three cases, strings can be interpreted either as one
character per code unit, or as UTF-encoded Unicode, with support for
Unicode general category properties. Unicode support is optional at
build time (but is the default). However, processing strings as UTF
code units must be enabled explicitly at run time. The version of Uni-
code in use can be discovered by running
pcre2test -C
The three libraries contain identical sets of functions, with names
ending in _8, _16, or _32, respectively (for example, pcre2_com-
pile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
32, a program that uses just one code unit width can be written using
generic names such as pcre2_compile(), and the documentation is written
assuming that this is the case.
In addition to the Perl-compatible matching function, PCRE2 contains an
alternative function that matches the same compiled patterns in a dif-
ferent way. In certain circumstances, the alternative function has some
advantages. For a discussion of the two matching algorithms, see the
pcre2matching page.
Details of exactly which Perl regular expression features are and are
not supported by PCRE2 are given in separate documents. See the
pcre2pattern and pcre2compat pages. There is a syntax summary in the
pcre2syntax page.
Some features of PCRE2 can be included, excluded, or changed when the
library is built. The pcre2_config() function makes it possible for a
client to discover which features are available. The features them-
selves are described in the pcre2build page. Documentation about build-
ing PCRE2 for various operating systems can be found in the README and
NON-AUTOTOOLS_BUILD files in the source distribution.
The libraries contains a number of undocumented internal functions and
data tables that are used by more than one of the exported external
functions, but which are not intended for use by external callers.
Their names all begin with "_pcre2", which hopefully will not provoke
any name clashes. In some environments, it is possible to control which
external symbols are exported when a shared library is built, and in
these cases the undocumented symbols are not exported.
SECURITY CONSIDERATIONS
If you are using PCRE2 in a non-UTF application that permits users to
supply arbitrary patterns for compilation, you should be aware of a
feature that allows users to turn on UTF support from within a pattern.
For example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8
mode, which interprets patterns and subjects as strings of UTF-8 code
units instead of individual 8-bit characters. This causes both the pat-
tern and any data against which it is matched to be checked for UTF-8
validity. If the data string is very long, such a check might use suf-
ficiently many resources as to cause your application to lose perfor-
mance.
One way of guarding against this possibility is to use the pcre2_pat-
tern_info() function to check the compiled pattern's options for
PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when
calling pcre2_compile(). This causes an compile time error if a pattern
contains a UTF-setting sequence.
The use of Unicode properties for character types such as \d can also
be enabled from within the pattern, by specifying "(*UCP)". This fea-
ture can be disallowed by setting the PCRE2_NEVER_UCP option.
If your application is one that supports UTF, be aware that validity
checking can take time. If the same data string is to be matched many
times, you can use the PCRE2_NO_UTF_CHECK option for the second and
subsequent matches to avoid running redundant checks.
The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
to problems, because it may leave the current matching point in the
middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C
option can be used to lock out the use of \C, causing a compile-time
error if it is encountered.
Another way that performance can be hit is by running a pattern that
has a very large search tree against a string that will never match.
Nested unlimited repeats in a pattern are a common example. PCRE2 pro-
vides some protection against this: see the pcre2_set_match_limit()
function in the pcre2api page.
USER DOCUMENTATION
The user documentation for PCRE2 comprises a number of different sec-
tions. In the "man" format, each of these is a separate "man page". In
the HTML format, each is a separate page, linked from the index page.
In the plain text format, the descriptions of the pcre2grep and
pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
respectively. The remaining sections, except for the pcre2demo section
(which is a program listing), and the short pages for individual func-
tions, are concatenated in pcre2.txt, for ease of searching. The sec-
tions are as follows:
pcre2 this document
pcre2-config show PCRE2 installation configuration information
pcre2api details of PCRE2's native C API
pcre2build building PCRE2
pcre2callout details of the callout feature
pcre2compat discussion of Perl compatibility
pcre2demo a demonstration C program that uses PCRE2
pcre2grep description of the pcre2grep command (8-bit only)
pcre2jit discussion of just-in-time optimization support
pcre2limits details of size and other limits
pcre2matching discussion of the two matching algorithms
pcre2partial details of the partial matching facility
pcre2pattern syntax and semantics of supported regular
expression patterns
pcre2perform discussion of performance issues
pcre2posix the POSIX-compatible C API for the 8-bit library
pcre2sample discussion of the pcre2demo program
pcre2stack discussion of stack usage
pcre2syntax quick syntax reference
pcre2test description of the pcre2test command
pcre2unicode discussion of Unicode and UTF support
In the "man" and HTML formats, there is also a short page for each C
library function, listing its arguments and results.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
Putting an actual email address here is a spam magnet. If you want to
email me, use my two initials, followed by the two digits 10, at the
domain cam.ac.uk.
REVISION
Last updated: 13 April 2015
Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------
PCRE2API(3) Library Functions Manual PCRE2API(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
#include <pcre2.h>
PCRE2 is a new API for PCRE. This document contains a description of
all its functions. See the pcre2 document for an overview of all the
PCRE2 documentation.
PCRE2 NATIVE API BASIC FUNCTIONS
pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
pcre2_compile_context *ccontext);
pcre2_code_free(pcre2_code *code);
pcre2_match_data_create(uint32_t ovecsize,
pcre2_general_context *gcontext);
pcre2_match_data_create_from_pattern(const pcre2_code *code,
pcre2_general_context *gcontext);
int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext);
int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext,
int *workspace, PCRE2_SIZE wscount);
void pcre2_match_data_free(pcre2_match_data *match_data);
PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS
PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);
uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);
PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);
PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS
pcre2_general_context *pcre2_general_context_create(
void *(*private_malloc)(PCRE2_SIZE, void *),
void (*private_free)(void *, void *), void *memory_data);
pcre2_general_context *pcre2_general_context_copy(
pcre2_general_context *gcontext);
void pcre2_general_context_free(pcre2_general_context *gcontext);
PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS
pcre2_compile_context *pcre2_compile_context_create(
pcre2_general_context *gcontext);
pcre2_compile_context *pcre2_compile_context_copy(
pcre2_compile_context *ccontext);
void pcre2_compile_context_free(pcre2_compile_context *ccontext);
int pcre2_set_bsr(pcre2_compile_context *ccontext,
uint32_t value);
int pcre2_set_character_tables(pcre2_compile_context *ccontext,
const unsigned char *tables);
int pcre2_set_newline(pcre2_compile_context *ccontext,
uint32_t value);
int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
uint32_t value);
int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
int (*guard_function)(uint32_t, void *), void *user_data);
PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS
pcre2_match_context *pcre2_match_context_create(
pcre2_general_context *gcontext);
pcre2_match_context *pcre2_match_context_copy(
pcre2_match_context *mcontext);
void pcre2_match_context_free(pcre2_match_context *mcontext);
int pcre2_set_callout(pcre2_match_context *mcontext,
int (*callout_function)(pcre2_callout_block *, void *),
void *callout_data);
int pcre2_set_match_limit(pcre2_match_context *mcontext,
uint32_t value);
int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
uint32_t value);
int pcre2_set_recursion_memory_management(
pcre2_match_context *mcontext,
void *(*private_malloc)(PCRE2_SIZE, void *),
void (*private_free)(void *, void *), void *memory_data);
PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS
int pcre2_substring_copy_byname(pcre2_match_data *match_data,
PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);
int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
uint32_t number, PCRE2_UCHAR *buffer,
PCRE2_SIZE *bufflen);
void pcre2_substring_free(PCRE2_UCHAR *buffer);
int pcre2_substring_get_byname(pcre2_match_data *match_data,
PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);
int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
uint32_t number, PCRE2_UCHAR **bufferptr,
PCRE2_SIZE *bufflen);
int pcre2_substring_length_byname(pcre2_match_data *match_data,
PCRE2_SPTR name, PCRE2_SIZE *length);
int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
uint32_t number, PCRE2_SIZE *length);
int pcre2_substring_nametable_scan(const pcre2_code *code,
PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);
int pcre2_substring_number_from_name(const pcre2_code *code,
PCRE2_SPTR name);
void pcre2_substring_list_free(PCRE2_SPTR *list);
int pcre2_substring_list_get(pcre2_match_data *match_data,
PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);
PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION
int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext, PCRE2_SPTR replacementzfP,
PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
PCRE2_SIZE *outlengthptr);
PCRE2 NATIVE API JIT FUNCTIONS
int pcre2_jit_compile(pcre2_code *code, uint32_t options);
int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext);
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize,
PCRE2_SIZE maxsize, pcre2_general_context *gcontext);
void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
pcre2_jit_callback callback_function, void *callback_data);
void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);
PCRE2 NATIVE API SERIALIZATION FUNCTIONS
int32_t pcre2_serialize_decode(pcre2_code **codes,
int32_t number_of_codes, const uint32_t *bytes,
pcre2_general_context *gcontext);
int32_t pcre2_serialize_encode(pcre2_code **codes,
int32_t number_of_codes, uint32_t **serialized_bytes,
PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);
void pcre2_serialize_free(uint8_t *bytes);
int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);
PCRE2 NATIVE API AUXILIARY FUNCTIONS
int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
PCRE2_SIZE bufflen);
const unsigned char *pcre2_maketables(pcre2_general_context *gcontext);
int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);
int pcre2_callout_enumerate(const pcre2_code *code,
int (*callback)(pcre2_callout_enumerate_block *, void *),
void *user_data);
int pcre2_config(uint32_t what, void *where);
PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES
There are three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit
code units, respectively. However, there is just one header file,
pcre2.h. This contains the function prototypes and other definitions
for all three libraries. One, two, or all three can be installed simul-
taneously. On Unix-like systems the libraries are called libpcre2-8,
libpcre2-16, and libpcre2-32, and they can also co-exist with the orig-
inal PCRE libraries.
Character strings are passed to and from a PCRE2 library as a sequence
of unsigned integers in code units of the appropriate width. Every
PCRE2 function comes in three different forms, one for each library,
for example:
pcre2_compile_8()
pcre2_compile_16()
pcre2_compile_32()
There are also three different sets of data types:
PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32
PCRE2_SPTR8, PCRE2_SPTR16, PCRE2_SPTR32
The UCHAR types define unsigned code units of the appropriate widths.
For example, PCRE2_UCHAR16 is usually defined as `uint16_t'. The SPTR
types are constant pointers to the equivalent UCHAR types, that is,
they are pointers to vectors of unsigned code units.
Many applications use only one code unit width. For their convenience,
macros are defined whose names are the generic forms such as pcre2_com-
pile() and PCRE2_SPTR. These macros use the value of the macro
PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific func-
tion and macro names. PCRE2_CODE_UNIT_WIDTH is not defined by default.
An application must define it to be 8, 16, or 32 before including
pcre2.h in order to make use of the generic names.
Applications that use more than one code unit width can be linked with
more than one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to
be 0 before including pcre2.h, and then use the real function names.
Any code that is to be included in an environment where the value of
PCRE2_CODE_UNIT_WIDTH is unknown should also use the real function
names. (Unfortunately, it is not possible in C code to save and restore
the value of a macro.)
If PCRE2_CODE_UNIT_WIDTH is not defined before including pcre2.h, a
compiler error occurs.
When using multiple libraries in an application, you must take care
when processing any particular pattern to use only functions from a
single library. For example, if you want to run a match using a pat-
tern that was compiled with pcre2_compile_16(), you must do so with
pcre2_match_16(), not pcre2_match_8().
In the function summaries above, and in the rest of this document and
other PCRE2 documents, functions and data types are described using
their generic names, without the 8, 16, or 32 suffix.
PCRE2 API OVERVIEW
PCRE2 has its own native API, which is described in this document.
There are also some wrapper functions for the 8-bit library that corre-
spond to the POSIX regular expression API, but they do not give access
to all the functionality. They are described in the pcre2posix documen-
tation. Both these APIs define a set of C function calls.
The native API C data types, function prototypes, option values, and
error codes are defined in the header file pcre2.h, which contains def-
initions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release
numbers for the library. Applications can use these to include support
for different releases of PCRE2.
In a Windows environment, if you want to statically link an application
program against a non-dll PCRE2 library, you must define PCRE2_STATIC
before including pcre2.h.
The functions pcre2_compile(), and pcre2_match() are used for compiling
and matching regular expressions in a Perl-compatible manner. A sample
program that demonstrates the simplest way of using them is provided in
the file called pcre2demo.c in the PCRE2 source distribution. A listing
of this program is given in the pcre2demo documentation, and the
pcre2sample documentation describes how to compile and run it.
Just-in-time compiler support is an optional feature of PCRE2 that can
be built in appropriate hardware environments. It greatly speeds up the
matching performance of many patterns. Programs can request that it be
used if available, by calling pcre2_jit_compile() after a pattern has
been successfully compiled by pcre2_compile(). This does nothing if JIT
support is not available.
More complicated programs might need to make use of the specialist
functions pcre2_jit_stack_create(), pcre2_jit_stack_free(), and
pcre2_jit_stack_assign() in order to control the JIT code's memory
usage.
JIT matching is automatically used by pcre2_match() if it is available.
There is also a direct interface for JIT matching, which gives improved
performance. The JIT-specific functions are discussed in the pcre2jit
documentation.
A second matching function, pcre2_dfa_match(), which is not Perl-com-
patible, is also provided. This uses a different algorithm for the
matching. The alternative algorithm finds all possible matches (at a
given point in the subject), and scans the subject just once (unless
there are lookbehind assertions). However, this algorithm does not
return captured substrings. A description of the two matching algo-
rithms and their advantages and disadvantages is given in the
pcre2matching documentation. There is no JIT support for
pcre2_dfa_match().
In addition to the main compiling and matching functions, there are
convenience functions for extracting captured substrings from a subject
string that has been matched by pcre2_match(). They are:
pcre2_substring_copy_byname()
pcre2_substring_copy_bynumber()
pcre2_substring_get_byname()
pcre2_substring_get_bynumber()
pcre2_substring_list_get()
pcre2_substring_length_byname()
pcre2_substring_length_bynumber()
pcre2_substring_nametable_scan()
pcre2_substring_number_from_name()
pcre2_substring_free() and pcre2_substring_list_free() are also pro-
vided, to free the memory used for extracted strings.
The function pcre2_substitute() can be called to match a pattern and
return a copy of the subject string with substitutions for parts that
were matched.
Finally, there are functions for finding out information about a com-
piled pattern (pcre2_pattern_info()) and about the configuration with
which PCRE2 was built (pcre2_config()).
STRING LENGTHS AND OFFSETS
The PCRE2 API uses string lengths and offsets into strings of code
units in several places. These values are always of type PCRE2_SIZE,
which is an unsigned integer type, currently always defined as size_t.
The largest value that can be stored in such a type (that is
~(PCRE2_SIZE)0) is reserved as a special indicator for zero-terminated
strings and unset offsets. Therefore, the longest string that can be
handled is one less than this maximum.
NEWLINES
PCRE2 supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (line-
feed) character, the two-character sequence CRLF, any of the three pre-
ceding, or any Unicode newline sequence. The Unicode newline sequences
are the three just mentioned, plus the single characters VT (vertical
tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
separator, U+2028), and PS (paragraph separator, U+2029).
Each of the first three conventions is used by at least one operating
system as its standard newline sequence. When PCRE2 is built, a default
can be specified. The default default is LF, which is the Unix stan-
dard. However, the newline convention can be changed by an application
when calling pcre2_compile(), or it can be specified by special text at
the start of the pattern itself; this overrides any other settings. See
the pcre2pattern page for details of the special character sequences.
In the PCRE2 documentation the word "newline" is used to mean "the
character or pair of characters that indicate a line break". The choice
of newline convention affects the handling of the dot, circumflex, and
dollar metacharacters, the handling of #-comments in /x mode, and, when
CRLF is a recognized line ending sequence, the match position advance-
ment for a non-anchored pattern. There is more detail about this in the
section on pcre2_match() options below.
The choice of newline convention does not affect the interpretation of
the \n or \r escape sequences, nor does it affect what \R matches; this
has its own separate convention.
MULTITHREADING
In a multithreaded application it is important to keep thread-specific
data separate from data that can be shared between threads. The PCRE2
library code itself is thread-safe: it contains no static or global
variables. The API is designed to be fairly simple for non-threaded
applications while at the same time ensuring that multithreaded appli-
cations can use it.
There are several different blocks of data that are used to pass infor-
mation between the application and the PCRE2 libraries.
(1) A pointer to the compiled form of a pattern is returned to the user
when pcre2_compile() is successful. The data in the compiled pattern is
fixed, and does not change when the pattern is matched. Therefore, it
is thread-safe, that is, the same compiled pattern can be used by more
than one thread simultaneously. An application can compile all its pat-
terns at the start, before forking off multiple threads that use them.
However, if the just-in-time optimization feature is being used, it
needs separate memory stack areas for each thread. See the pcre2jit
documentation for more details.
(2) The next section below introduces the idea of "contexts" in which
PCRE2 functions are called. A context is nothing more than a collection
of parameters that control the way PCRE2 operates. Grouping a number of
parameters together in a context is a convenient way of passing them to
a PCRE2 function without using lots of arguments. The parameters that
are stored in contexts are in some sense "advanced features" of the
API. Many straightforward applications will not need to use contexts.
In a multithreaded application, if the parameters in a context are val-
ues that are never changed, the same context can be used by all the
threads. However, if any thread needs to change any value in a context,
it must make its own thread-specific copy.
(3) The matching functions need a block of memory for working space and
for storing the results of a match. This includes details of what was
matched, as well as additional information such as the name of a
(*MARK) setting. Each thread must provide its own version of this mem-
ory.
PCRE2 CONTEXTS
Some PCRE2 functions have a lot of parameters, many of which are used
only by specialist applications, for example, those that use custom
memory management or non-standard character tables. To keep function
argument lists at a reasonable size, and at the same time to keep the
API extensible, "uncommon" parameters are passed to certain functions
in a context instead of directly. A context is just a block of memory
that holds the parameter values. Applications that do not need to
adjust any of the context parameters can pass NULL when a context
pointer is required.
There are three different types of context: a general context that is
relevant for several PCRE2 operations, a compile-time context, and a
match-time context.
The general context
At present, this context just contains pointers to (and data for)
external memory management functions that are called from several
places in the PCRE2 library. The context is named `general' rather than
specifically `memory' because in future other fields may be added. If
you do not want to supply your own custom memory management functions,
you do not need to bother with a general context. A general context is
created by:
pcre2_general_context *pcre2_general_context_create(
void *(*private_malloc)(PCRE2_SIZE, void *),
void (*private_free)(void *, void *), void *memory_data);
The two function pointers specify custom memory management functions,
whose prototypes are:
void *private_malloc(PCRE2_SIZE, void *);
void private_free(void *, void *);
Whenever code in PCRE2 calls these functions, the final argument is the
value of memory_data. Either of the first two arguments of the creation
function may be NULL, in which case the system memory management func-
tions malloc() and free() are used. (This is not currently useful, as
there are no other fields in a general context, but in future there
might be.) The private_malloc() function is used (if supplied) to
obtain memory for storing the context, and all three values are saved
as part of the context.
Whenever PCRE2 creates a data block of any kind, the block contains a
pointer to the free() function that matches the malloc() function that
was used. When the time comes to free the block, this function is
called.
A general context can be copied by calling:
pcre2_general_context *pcre2_general_context_copy(
pcre2_general_context *gcontext);
The memory used for a general context should be freed by calling:
void pcre2_general_context_free(pcre2_general_context *gcontext);
The compile context
A compile context is required if you want to change the default values
of any of the following compile-time parameters:
What \R matches (Unicode newlines or CR, LF, CRLF only)
PCRE2's character tables
The newline character sequence
The compile time nested parentheses limit
An external function for stack checking
A compile context is also required if you are using custom memory man-
agement. If none of these apply, just pass NULL as the context argu-
ment of pcre2_compile().
A compile context is created, copied, and freed by the following func-
tions:
pcre2_compile_context *pcre2_compile_context_create(
pcre2_general_context *gcontext);
pcre2_compile_context *pcre2_compile_context_copy(
pcre2_compile_context *ccontext);
void pcre2_compile_context_free(pcre2_compile_context *ccontext);
A compile context is created with default values for its parameters.
These can be changed by calling the following functions, which return 0
on success, or PCRE2_ERROR_BADDATA if invalid data is detected.
int pcre2_set_bsr(pcre2_compile_context *ccontext,
uint32_t value);
The value must be PCRE2_BSR_ANYCRLF, to specify that \R matches only
CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any
Unicode line ending sequence. The value is used by the JIT compiler and
by the two interpreted matching functions, pcre2_match() and
pcre2_dfa_match().
int pcre2_set_character_tables(pcre2_compile_context *ccontext,
const unsigned char *tables);
The value must be the result of a call to pcre2_maketables(), whose
only argument is a general context. This function builds a set of char-
acter tables in the current locale.
int pcre2_set_newline(pcre2_compile_context *ccontext,
uint32_t value);
This specifies which characters or character sequences are to be recog-
nized as newlines. The value must be one of PCRE2_NEWLINE_CR (carriage
return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the
two-character sequence CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any
of the above), or PCRE2_NEWLINE_ANY (any Unicode newline sequence).
When a pattern is compiled with the PCRE2_EXTENDED option, the value of
this parameter affects the recognition of white space and the end of
internal comments starting with #. The value is saved with the compiled
pattern for subsequent use by the JIT compiler and by the two inter-
preted matching functions, pcre2_match() and pcre2_dfa_match().
int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
uint32_t value);
This parameter ajusts the limit, set when PCRE2 is built (default 250),
on the depth of parenthesis nesting in a pattern. This limit stops
rogue patterns using up too much system stack when being compiled.
int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
int (*guard_function)(uint32_t, void *), void *user_data);
There is at least one application that runs PCRE2 in threads with very
limited system stack, where running out of stack is to be avoided at
all costs. The parenthesis limit above cannot take account of how much
stack is actually available. For a finer control, you can supply a
function that is called whenever pcre2_compile() starts to compile a
parenthesized part of a pattern. This function can check the actual
stack size (or anything else that it wants to, of course).
The first argument to the callout function gives the current depth of
nesting, and the second is user data that is set up by the last argu-
ment of pcre2_set_compile_recursion_guard(). The callout function
should return zero if all is well, or non-zero to force an error.
The match context
A match context is required if you want to change the default values of
any of the following match-time parameters:
A callout function
The limit for calling match()
The limit for calling match() recursively
A match context is also required if you are using custom memory manage-
ment. If none of these apply, just pass NULL as the context argument
of pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().
A match context is created, copied, and freed by the following func-
tions:
pcre2_match_context *pcre2_match_context_create(
pcre2_general_context *gcontext);
pcre2_match_context *pcre2_match_context_copy(
pcre2_match_context *mcontext);
void pcre2_match_context_free(pcre2_match_context *mcontext);
A match context is created with default values for its parameters.
These can be changed by calling the following functions, which return 0
on success, or PCRE2_ERROR_BADDATA if invalid data is detected.
int pcre2_set_callout(pcre2_match_context *mcontext,
int (*callout_function)(pcre2_callout_block *, void *),
void *callout_data);
This sets up a "callout" function, which PCRE2 will call at specified
points during a matching operation. Details are given in the pcre2call-
out documentation.
int pcre2_set_match_limit(pcre2_match_context *mcontext,
uint32_t value);
The match_limit parameter provides a means of preventing PCRE2 from
using up too many resources when processing patterns that are not going
to match, but which have a very large number of possibilities in their
search trees. The classic example is a pattern that uses nested unlim-
ited repeats.
Internally, pcre2_match() uses a function called match(), which it
calls repeatedly (sometimes recursively). The limit set by match_limit
is imposed on the number of times this function is called during a
match, which has the effect of limiting the amount of backtracking that
can take place. For patterns that are not anchored, the count restarts
from zero for each position in the subject string. This limit is not
relevant to pcre2_dfa_match(), which ignores it.
When pcre2_match() is called with a pattern that was successfully pro-
cessed by pcre2_jit_compile(), the way in which matching is executed is
entirely different. However, there is still the possibility of runaway
matching that goes on for a very long time, and so the match_limit
value is also used in this case (but in a different way) to limit how
long the matching can continue.
The default value for the limit can be set when PCRE2 is built; the
default default is 10 million, which handles all but the most extreme
cases. If the limit is exceeded, pcre2_match() returns
PCRE2_ERROR_MATCHLIMIT. A value for the match limit may also be sup-
plied by an item at the start of a pattern of the form
(*LIMIT_MATCH=ddd)
where ddd is a decimal number. However, such a setting is ignored
unless ddd is less than the limit set by the caller of pcre2_match()
or, if no such limit is set, less than the default.
int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
uint32_t value);
The recursion_limit parameter is similar to match_limit, but instead of
limiting the total number of times that match() is called, it limits
the depth of recursion. The recursion depth is a smaller number than
the total number of calls, because not all calls to match() are recur-
sive. This limit is of use only if it is set smaller than match_limit.
Limiting the recursion depth limits the amount of system stack that can
be used, or, when PCRE2 has been compiled to use memory on the heap
instead of the stack, the amount of heap memory that can be used. This
limit is not relevant, and is ignored, when matching is done using JIT
compiled code or by the pcre2_dfa_match() function.
The default value for recursion_limit can be set when PCRE2 is built;
the default default is the same value as the default for match_limit.
If the limit is exceeded, pcre2_match() returns PCRE2_ERROR_RECURSION-
LIMIT. A value for the recursion limit may also be supplied by an item
at the start of a pattern of the form
(*LIMIT_RECURSION=ddd)
where ddd is a decimal number. However, such a setting is ignored
unless ddd is less than the limit set by the caller of pcre2_match()
or, if no such limit is set, less than the default.
int pcre2_set_recursion_memory_management(
pcre2_match_context *mcontext,
void *(*private_malloc)(PCRE2_SIZE, void *),
void (*private_free)(void *, void *), void *memory_data);
This function sets up two additional custom memory management functions
for use by pcre2_match() when PCRE2 is compiled to use the heap for
remembering backtracking data, instead of recursive function calls that
use the system stack. There is a discussion about PCRE2's stack usage
in the pcre2stack documentation. See the pcre2build documentation for
details of how to build PCRE2.
Using the heap for recursion is a non-standard way of building PCRE2,
for use in environments that have limited stacks. Because of the
greater use of memory management, pcre2_match() runs more slowly. Func-
tions that are different to the general custom memory functions are
provided so that special-purpose external code can be used for this
case, because the memory blocks are all the same size. The blocks are
retained by pcre2_match() until it is about to exit so that they can be
re-used when possible during the match. In the absence of these func-
tions, the normal custom memory management functions are used, if sup-
plied, otherwise the system functions.
CHECKING BUILD-TIME OPTIONS
int pcre2_config(uint32_t what, void *where);
The function pcre2_config() makes it possible for a PCRE2 client to
discover which optional features have been compiled into the PCRE2
library. The pcre2build documentation has more details about these
optional features.
The first argument for pcre2_config() specifies which information is
required. The second argument is a pointer to memory into which the
information is placed. If NULL is passed, the function returns the
amount of memory that is needed for the requested information. For
calls that return numerical values, the value is in bytes; when
requesting these values, where should point to appropriately aligned
memory. For calls that return strings, the required length is given in
code units, not counting the terminating zero.
When requesting information, the returned value from pcre2_config() is
non-negative on success, or the negative error code PCRE2_ERROR_BADOP-
TION if the value in the first argument is not recognized. The follow-
ing information is available:
PCRE2_CONFIG_BSR
The output is a uint32_t integer whose value indicates what character
sequences the \R escape sequence matches by default. A value of
PCRE2_BSR_UNICODE means that \R matches any Unicode line ending
sequence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR,
LF, or CRLF. The default can be overridden when a pattern is compiled.
PCRE2_CONFIG_JIT
The output is a uint32_t integer that is set to one if support for
just-in-time compiling is available; otherwise it is set to zero.
PCRE2_CONFIG_JITTARGET
The where argument should point to a buffer that is at least 48 code
units long. (The exact length required can be found by calling
pcre2_config() with where set to NULL.) The buffer is filled with a
string that contains the name of the architecture for which the JIT
compiler is configured, for example "x86 32bit (little endian +
unaligned)". If JIT support is not available, PCRE2_ERROR_BADOPTION is
returned, otherwise the number of code units used is returned. This is
the length of the string, plus one unit for the terminating zero.
PCRE2_CONFIG_LINKSIZE
The output is a uint32_t integer that contains the number of bytes used
for internal linkage in compiled regular expressions. When PCRE2 is
configured, the value can be set to 2, 3, or 4, with the default being
2. This is the value that is returned by pcre2_config(). However, when
the 16-bit library is compiled, a value of 3 is rounded up to 4, and
when the 32-bit library is compiled, internal linkages always use 4
bytes, so the configured value is not relevant.
The default value of 2 for the 8-bit and 16-bit libraries is sufficient
for all but the most massive patterns, since it allows the size of the
compiled pattern to be up to 64K code units. Larger values allow larger
regular expressions to be compiled by those two libraries, but at the
expense of slower matching.
PCRE2_CONFIG_MATCHLIMIT
The output is a uint32_t integer that gives the default limit for the
number of internal matching function calls in a pcre2_match() execu-
tion. Further details are given with pcre2_match() below.
PCRE2_CONFIG_NEWLINE
The output is a uint32_t integer whose value specifies the default
character sequence that is recognized as meaning "newline". The values
are:
PCRE2_NEWLINE_CR Carriage return (CR)
PCRE2_NEWLINE_LF Linefeed (LF)
PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF)
PCRE2_NEWLINE_ANY Any Unicode line ending
PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF
The default should normally correspond to the standard sequence for
your operating system.
PCRE2_CONFIG_PARENSLIMIT
The output is a uint32_t integer that gives the maximum depth of nest-
ing of parentheses (of any kind) in a pattern. This limit is imposed to
cap the amount of system stack used when a pattern is compiled. It is
specified when PCRE2 is built; the default is 250. This limit does not
take into account the stack that may already be used by the calling
application. For finer control over compilation stack usage, see
pcre2_set_compile_recursion_guard().
PCRE2_CONFIG_RECURSIONLIMIT
The output is a uint32_t integer that gives the default limit for the
depth of recursion when calling the internal matching function in a
pcre2_match() execution. Further details are given with pcre2_match()
below.
PCRE2_CONFIG_STACKRECURSE
The output is a uint32_t integer that is set to one if internal recur-
sion when running pcre2_match() is implemented by recursive function
calls that use the system stack to remember their state. This is the
usual way that PCRE2 is compiled. The output is zero if PCRE2 was com-
piled to use blocks of data on the heap instead of recursive function
calls.
PCRE2_CONFIG_UNICODE_VERSION
The where argument should point to a buffer that is at least 24 code
units long. (The exact length required can be found by calling
pcre2_config() with where set to NULL.) If PCRE2 has been compiled
without Unicode support, the buffer is filled with the text "Unicode
not supported". Otherwise, the Unicode version string (for example,
"7.0.0") is inserted. The number of code units used is returned. This
is the length of the string plus one unit for the terminating zero.
PCRE2_CONFIG_UNICODE
The output is a uint32_t integer that is set to one if Unicode support
is available; otherwise it is set to zero. Unicode support implies UTF
support.
PCRE2_CONFIG_VERSION
The where argument should point to a buffer that is at least 12 code
units long. (The exact length required can be found by calling
pcre2_config() with where set to NULL.) The buffer is filled with the
PCRE2 version string, zero-terminated. The number of code units used is
returned. This is the length of the string plus one unit for the termi-
nating zero.
COMPILING A PATTERN
pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
pcre2_compile_context *ccontext);
pcre2_code_free(pcre2_code *code);
The pcre2_compile() function compiles a pattern into an internal form.
The pattern is defined by a pointer to a string of code units and a
length, If the pattern is zero-terminated, the length can be specified
as PCRE2_ZERO_TERMINATED. The function returns a pointer to a block of
memory that contains the compiled pattern and related data. The caller
must free the memory by calling pcre2_code_free() when it is no longer
needed.
NOTE: When one of the matching functions is called, pointers to the
compiled pattern and the subject string are set in the match data block
so that they can be referenced by the extraction functions. After run-
ning a match, you must not free a compiled pattern (or a subject
string) until after all operations on the match data block have taken
place.
If the compile context argument ccontext is NULL, memory for the com-
piled pattern is obtained by calling malloc(). Otherwise, it is
obtained from the same memory function that was used for the compile
context.
The options argument contains various bit settings that affect the com-
pilation. It should be zero if no options are required. The available
options are described below. Some of them (in particular, those that
are compatible with Perl, but some others as well) can also be set and
unset from within the pattern (see the detailed description in the
pcre2pattern documentation).
For those options that can be different in different parts of the pat-
tern, the contents of the options argument specifies their settings at
the start of compilation. The PCRE2_ANCHORED and PCRE2_NO_UTF_CHECK
options can be set at the time of matching as well as at compile time.
Other, less frequently required compile-time parameters (for example,
the newline setting) can be provided in a compile context (as described
above).
If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme-
diately. Otherwise, if compilation of a pattern fails, pcre2_compile()
returns NULL, having set these variables to an error code and an offset
(number of code units) within the pattern, respectively. The
pcre2_get_error_message() function provides a textual message for each
error code. Compilation errors are positive numbers, but UTF formatting
errors are negative numbers. For an invalid UTF-8 or UTF-16 string, the
offset is that of the first code unit of the failing character.
Some errors are not detected until the whole pattern has been scanned;
in these cases, the offset passed back is the length of the pattern.
Note that the offset is in code units, not characters, even in a UTF
mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char-
acter.
This code fragment shows a typical straightforward call to pcre2_com-
pile():
pcre2_code *re;
PCRE2_SIZE erroffset;
int errorcode;
re = pcre2_compile(
"^A.*Z", /* the pattern */
PCRE2_ZERO_TERMINATED, /* the pattern is zero-terminated */
0, /* default options */
&errorcode, /* for error code */
&erroffset, /* for error offset */
NULL); /* no compile context */
The following names for option bits are defined in the pcre2.h header
file:
PCRE2_ANCHORED
If this bit is set, the pattern is forced to be "anchored", that is, it
is constrained to match only at the first matching point in the string
that is being searched (the "subject string"). This effect can also be
achieved by appropriate constructs in the pattern itself, which is the
only way to do it in Perl.
PCRE2_ALLOW_EMPTY_CLASS
By default, for compatibility with Perl, a closing square bracket that
immediately follows an opening one is treated as a data character for
the class. When PCRE2_ALLOW_EMPTY_CLASS is set, it terminates the
class, which therefore contains no characters and so can never match.
PCRE2_ALT_BSUX
This option request alternative handling of three escape sequences,
which makes PCRE2's behaviour more like ECMAscript (aka JavaScript).
When it is set:
(1) \U matches an upper case "U" character; by default \U causes a com-
pile time error (Perl uses \U to upper case subsequent characters).
(2) \u matches a lower case "u" character unless it is followed by four
hexadecimal digits, in which case the hexadecimal number defines the
code point to match. By default, \u causes a compile time error (Perl
uses it to upper case the following character).
(3) \x matches a lower case "x" character unless it is followed by two
hexadecimal digits, in which case the hexadecimal number defines the
code point to match. By default, as in Perl, a hexadecimal number is
always expected after \x, but it may have zero, one, or two digits (so,
for example, \xz matches a binary zero character followed by z).
PCRE2_ALT_CIRCUMFLEX
In multiline mode (when PCRE2_MULTILINE is set), the circumflex
metacharacter matches at the start of the subject (unless PCRE2_NOTBOL
is set), and also after any internal newline. However, it does not
match after a newline at the end of the subject, for compatibility with
Perl. If you want a multiline circumflex also to match after a termi-
nating newline, you must set PCRE2_ALT_CIRCUMFLEX.
PCRE2_AUTO_CALLOUT
If this bit is set, pcre2_compile() automatically inserts callout
items, all with number 255, before each pattern item. For discussion of
the callout facility, see the pcre2callout documentation.
PCRE2_CASELESS
If this bit is set, letters in the pattern match both upper and lower
case letters in the subject. It is equivalent to Perl's /i option, and
it can be changed within a pattern by a (?i) option setting.
PCRE2_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern matches only
at the end of the subject string. Without this option, a dollar also
matches immediately before a newline at the end of the string (but not
before any other newlines). The PCRE2_DOLLAR_ENDONLY option is ignored
if PCRE2_MULTILINE is set. There is no equivalent to this option in
Perl, and no way to set it within a pattern.
PCRE2_DOTALL
If this bit is set, a dot metacharacter in the pattern matches any
character, including one that indicates a newline. However, it only
ever matches one character, even if newlines are coded as CRLF. Without
this option, a dot does not match when the current position in the sub-
ject is at a newline. This option is equivalent to Perl's /s option,
and it can be changed within a pattern by a (?s) option setting. A neg-
ative class such as [^a] always matches newline characters, independent
of the setting of this option.
PCRE2_DUPNAMES
If this bit is set, names used to identify capturing subpatterns need
not be unique. This can be helpful for certain types of pattern when it
is known that only one instance of the named subpattern can ever be
matched. There are more details of named subpatterns below; see also
the pcre2pattern documentation.
PCRE2_EXTENDED
If this bit is set, most white space characters in the pattern are
totally ignored except when escaped or inside a character class. How-
ever, white space is not allowed within sequences such as (?> that
introduce various parenthesized subpatterns, nor within numerical quan-
tifiers such as {1,3}. Ignorable white space is permitted between an
item and a following quantifier and between a quantifier and a follow-
ing + that indicates possessiveness.
PCRE2_EXTENDED also causes characters between an unescaped # outside a
character class and the next newline, inclusive, to be ignored, which
makes it possible to include comments inside complicated patterns. Note
that the end of this type of comment is a literal newline sequence in
the pattern; escape sequences that happen to represent a newline do not
count. PCRE2_EXTENDED is equivalent to Perl's /x option, and it can be
changed within a pattern by a (?x) option setting.
Which characters are interpreted as newlines can be specified by a set-
ting in the compile context that is passed to pcre2_compile() or by a
special sequence at the start of the pattern, as described in the sec-
tion entitled "Newline conventions" in the pcre2pattern documentation.
A default is defined when PCRE2 is built.
PCRE2_FIRSTLINE
If this option is set, an unanchored pattern is required to match
before or at the first newline in the subject string, though the
matched text may continue over the newline.
PCRE2_MATCH_UNSET_BACKREF
If this option is set, a back reference to an unset subpattern group
matches an empty string (by default this causes the current matching
alternative to fail). A pattern such as (\1)(a) succeeds when this
option is set (assuming it can find an "a" in the subject), whereas it
fails by default, for Perl compatibility. Setting this option makes
PCRE2 behave more like ECMAscript (aka JavaScript).
PCRE2_MULTILINE
By default, for the purposes of matching "start of line" and "end of
line", PCRE2 treats the subject string as consisting of a single line
of characters, even if it actually contains newlines. The "start of
line" metacharacter (^) matches only at the start of the string, and
the "end of line" metacharacter ($) matches only at the end of the
string, or before a terminating newline (except when PCRE2_DOL-
LAR_ENDONLY is set). Note, however, that unless PCRE2_DOTALL is set,
the "any character" metacharacter (.) does not match at a newline. This
behaviour (for ^, $, and dot) is the same as Perl.
When PCRE2_MULTILINE it is set, the "start of line" and "end of line"
constructs match immediately following or immediately before internal
newlines in the subject string, respectively, as well as at the very
start and end. This is equivalent to Perl's /m option, and it can be
changed within a pattern by a (?m) option setting. Note that the "start
of line" metacharacter does not match after a newline at the end of the
subject, for compatibility with Perl. However, you can change this by
setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in a
subject string, or no occurrences of ^ or $ in a pattern, setting
PCRE2_MULTILINE has no effect.
PCRE2_NEVER_BACKSLASH_C
This option locks out the use of \C in the pattern that is being com-
piled. This escape can cause unpredictable behaviour in UTF-8 or
UTF-16 modes, because it may leave the current matching point in the
middle of a multi-code-unit character. This option may be useful in
applications that process patterns from external sources.
PCRE2_NEVER_UCP
This option locks out the use of Unicode properties for handling \B,
\b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as
described for the PCRE2_UCP option below. In particular, it prevents
the creator of the pattern from enabling this facility by starting the
pattern with (*UCP). This option may be useful in applications that
process patterns from external sources. The option combination PCRE_UCP
and PCRE_NEVER_UCP causes an error.
PCRE2_NEVER_UTF
This option locks out interpretation of the pattern as UTF-8, UTF-16,
or UTF-32, depending on which library is in use. In particular, it pre-
vents the creator of the pattern from switching to UTF interpretation
by starting the pattern with (*UTF). This option may be useful in
applications that process patterns from external sources. The combina-
tion of PCRE2_UTF and PCRE2_NEVER_UTF causes an error.
PCRE2_NO_AUTO_CAPTURE
If this option is set, it disables the use of numbered capturing paren-
theses in the pattern. Any opening parenthesis that is not followed by
? behaves as if it were followed by ?: but named parentheses can still
be used for capturing (and they acquire numbers in the usual way).
There is no equivalent of this option in Perl.
PCRE2_NO_AUTO_POSSESS
If this option is set, it disables "auto-possessification", which is an
optimization that, for example, turns a+b into a++b in order to avoid
backtracks into a+ that can never be successful. However, if callouts
are in use, auto-possessification means that some callouts are never
taken. You can set this option if you want the matching functions to do
a full unoptimized search and run all the callouts, but it is mainly
provided for testing purposes.
PCRE2_NO_DOTSTAR_ANCHOR
If this option is set, it disables an optimization that is applied when
.* is the first significant item in a top-level branch of a pattern,
and all the other branches also start with .* or with \A or \G or ^.
The optimization is automatically disabled for .* if it is inside an
atomic group or a capturing group that is the subject of a back refer-
ence, or if the pattern contains (*PRUNE) or (*SKIP). When the opti-
mization is not disabled, such a pattern is automatically anchored if
PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set
for any ^ items. Otherwise, the fact that any match must start either
at the start of the subject or following a newline is remembered. Like
other optimizations, this can cause callouts to be skipped.
PCRE2_NO_START_OPTIMIZE
This is an option whose main effect is at matching time. It does not
change what pcre2_compile() generates, but it does affect the output of
the JIT compiler.
There are a number of optimizations that may occur at the start of a
match, in order to speed up the process. For example, if it is known
that an unanchored match must start with a specific character, the
matching code searches the subject for that character, and fails imme-
diately if it cannot find it, without actually running the main match-
ing function. This means that a special item such as (*COMMIT) at the
start of a pattern is not considered until after a suitable starting
point for the match has been found. Also, when callouts or (*MARK)
items are in use, these "start-up" optimizations can cause them to be
skipped if the pattern is never actually used. The start-up optimiza-
tions are in effect a pre-scan of the subject that takes place before
the pattern is run.
The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations,
possibly causing performance to suffer, but ensuring that in cases
where the result is "no match", the callouts do occur, and that items
such as (*COMMIT) and (*MARK) are considered at every possible starting
position in the subject string.
Setting PCRE2_NO_START_OPTIMIZE may change the outcome of a matching
operation. Consider the pattern
(*COMMIT)ABC
When this is compiled, PCRE2 records the fact that a match must start
with the character "A". Suppose the subject string is "DEFABC". The
start-up optimization scans along the subject, finds "A" and runs the
first match attempt from there. The (*COMMIT) item means that the pat-
tern must match the current starting position, which in this case, it
does. However, if the same match is run with PCRE2_NO_START_OPTIMIZE
set, the initial scan along the subject string does not happen. The
first match attempt is run starting from "D" and when this fails,
(*COMMIT) prevents any further matches being tried, so the overall
result is "no match". There are also other start-up optimizations. For
example, a minimum length for the subject may be recorded. Consider the
pattern
(*MARK:A)(X|Y)
The minimum length for a match is one character. If the subject is
"ABC", there will be attempts to match "ABC", "BC", and "C". An attempt
to match an empty string at the end of the subject does not take place,
because PCRE2 knows that the subject is now too short, and so the
(*MARK) is never encountered. In this case, the optimization does not
affect the overall match result, which is still "no match", but it does
affect the auxiliary information that is returned.
PCRE2_NO_UTF_CHECK
When PCRE2_UTF is set, the validity of the pattern as a UTF string is
automatically checked. There are discussions about the validity of
UTF-8 strings, UTF-16 strings, and UTF-32 strings in the pcre2unicode
document. If an invalid UTF sequence is found, pcre2_compile() returns
a negative error code.
If you know that your pattern is valid, and you want to skip this check
for performance reasons, you can set the PCRE2_NO_UTF_CHECK option.
When it is set, the effect of passing an invalid UTF string as a pat-
tern is undefined. It may cause your program to crash or loop. Note
that this option can also be passed to pcre2_match() and
pcre_dfa_match(), to suppress validity checking of the subject string.
PCRE2_UCP
This option changes the way PCRE2 processes \B, \b, \D, \d, \S, \s, \W,
\w, and some of the POSIX character classes. By default, only ASCII
characters are recognized, but if PCRE2_UCP is set, Unicode properties
are used instead to classify characters. More details are given in the
section on generic character types in the pcre2pattern page. If you set
PCRE2_UCP, matching one of the items it affects takes much longer. The
option is available only if PCRE2 has been compiled with Unicode sup-
port.
PCRE2_UNGREEDY
This option inverts the "greediness" of the quantifiers so that they
are not greedy by default, but become greedy if followed by "?". It is
not compatible with Perl. It can also be set by a (?U) option setting
within the pattern.
PCRE2_UTF
This option causes PCRE2 to regard both the pattern and the subject
strings that are subsequently processed as strings of UTF characters
instead of single-code-unit strings. It is available when PCRE2 is
built to include Unicode support (which is the default). If Unicode
support is not available, the use of this option provokes an error.
Details of how this option changes the behaviour of PCRE2 are given in
the pcre2unicode page.
COMPILATION ERROR CODES
There are over 80 positive error codes that pcre2_compile() may return
if it finds an error in the pattern. There are also some negative error
codes that are used for invalid UTF strings. These are the same as
given by pcre2_match() and pcre2_dfa_match(), and are described in the
pcre2unicode page. The pcre2_get_error_message() function can be called
to obtain a textual error message from any error code.
JUST-IN-TIME (JIT) COMPILATION
int pcre2_jit_compile(pcre2_code *code, uint32_t options);
int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext);
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize,
PCRE2_SIZE maxsize, pcre2_general_context *gcontext);
void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
pcre2_jit_callback callback_function, void *callback_data);
void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);
These functions provide support for JIT compilation, which, if the
just-in-time compiler is available, further processes a compiled pat-
tern into machine code that executes much faster than the pcre2_match()
interpretive matching function. Full details are given in the pcre2jit
documentation.
JIT compilation is a heavyweight optimization. It can take some time
for patterns to be analyzed, and for one-off matches and simple pat-
terns the benefit of faster execution might be offset by a much slower
compilation time. Most, but not all patterns can be optimized by the
JIT compiler.
LOCALE SUPPORT
PCRE2 handles caseless matching, and determines whether characters are
letters, digits, or whatever, by reference to a set of tables, indexed
by character code point. This applies only to characters whose code
points are less than 256. By default, higher-valued code points never
match escapes such as \w or \d. However, if PCRE2 is built with UTF
support, all characters can be tested with \p and \P, or, alterna-
tively, the PCRE2_UCP option can be set when a pattern is compiled;
this causes \w and friends to use Unicode property support instead of
the built-in tables.
The use of locales with Unicode is discouraged. If you are handling
characters with code points greater than 128, you should either use
Unicode support, or use locales, but not try to mix the two.
PCRE2 contains an internal set of character tables that are used by
default. These are sufficient for many applications. Normally, the
internal tables recognize only ASCII characters. However, when PCRE2 is
built, it is possible to cause the internal tables to be rebuilt in the
default "C" locale of the local system, which may cause them to be dif-
ferent.
The internal tables can be overridden by tables supplied by the appli-
cation that calls PCRE2. These may be created in a different locale
from the default. As more and more applications change to using Uni-
code, the need for this locale support is expected to die away.
External tables are built by calling the pcre2_maketables() function,
in the relevant locale. The result can be passed to pcre2_compile() as
often as necessary, by creating a compile context and calling
pcre2_set_character_tables() to set the tables pointer therein. For
example, to build and use tables that are appropriate for the French
locale (where accented characters with values greater than 128 are
treated as letters), the following code could be used:
setlocale(LC_CTYPE, "fr_FR");
tables = pcre2_maketables(NULL);
ccontext = pcre2_compile_context_create(NULL);
pcre2_set_character_tables(ccontext, tables);
re = pcre2_compile(..., ccontext);
The locale name "fr_FR" is used on Linux and other Unix-like systems;
if you are using Windows, the name for the French locale is "french".
It is the caller's responsibility to ensure that the memory containing
the tables remains available for as long as it is needed.
The pointer that is passed (via the compile context) to pcre2_compile()
is saved with the compiled pattern, and the same tables are used by
pcre2_match() and pcre_dfa_match(). Thus, for any single pattern, com-
pilation, and matching all happen in the same locale, but different
patterns can be processed in different locales.
INFORMATION ABOUT A COMPILED PATTERN
int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);
The pcre2_pattern_info() function returns general information about a
compiled pattern. For information about callouts, see the next section.
The first argument for pcre2_pattern_info() is a pointer to the com-
piled pattern. The second argument specifies which piece of information
is required, and the third argument is a pointer to a variable to
receive the data. If the third argument is NULL, the first argument is
ignored, and the function returns the size in bytes of the variable
that is required for the information requested. Otherwise, The yield of
the function is zero for success, or one of the following negative num-
bers:
PCRE2_ERROR_NULL the argument code was NULL
PCRE2_ERROR_BADMAGIC the "magic number" was not found
PCRE2_ERROR_BADOPTION the value of what was invalid
PCRE2_ERROR_UNSET the requested field is not set
The "magic number" is placed at the start of each compiled pattern as
an simple check against passing an arbitrary memory pointer. Here is a
typical call of pcre2_pattern_info(), to obtain the length of the com-
piled pattern:
int rc;
size_t length;
rc = pcre2_pattern_info(
re, /* result of pcre2_compile() */
PCRE2_INFO_SIZE, /* what is required */
&length); /* where to put the data */
The possible values for the second argument are defined in pcre2.h, and
are as follows:
PCRE2_INFO_ALLOPTIONS
PCRE2_INFO_ARGOPTIONS
Return a copy of the pattern's options. The third argument should point
to a uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the
options that were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP-
TIONS returns the compile options as modified by any top-level option
settings at the start of the pattern itself. In other words, they are
the options that will be in force when matching starts. For example, if
the pattern /(?im)abc(?-i)d/ is compiled with the PCRE2_EXTENDED
option, the result is PCRE2_CASELESS, PCRE2_MULTILINE, and
PCRE2_EXTENDED.
A pattern compiled without PCRE2_ANCHORED is automatically anchored by
PCRE2 if the first significant item in every top-level branch is one of
the following:
^ unless PCRE2_MULTILINE is set
\A always
\G always
.* sometimes - see below
When .* is the first significant item, anchoring is possible only when
all the following are true:
.* is not in an atomic group
.* is not in a capturing group that is the subject
of a back reference
PCRE2_DOTALL is in force for .*
Neither (*PRUNE) nor (*SKIP) appears in the pattern.
PCRE2_NO_DOTSTAR_ANCHOR is not set.
For patterns that are auto-anchored, the PCRE2_ANCHORED bit is set in
the options returned for PCRE2_INFO_ALLOPTIONS.
PCRE2_INFO_BACKREFMAX
Return the number of the highest back reference in the pattern. The
third argument should point to an uint32_t variable. Named subpatterns
acquire numbers as well as names, and these count towards the highest
back reference. Back references such as \4 or \g{12} match the cap-
tured characters of the given group, but in addition, the check that a
capturing group is set in a conditional subpattern such as (?(3)a|b) is
also a back reference. Zero is returned if there are no back refer-
ences.
PCRE2_INFO_BSR
The output is a uint32_t whose value indicates what character sequences
the \R escape sequence matches. A value of PCRE2_BSR_UNICODE means that
\R matches any Unicode line ending sequence; a value of PCRE2_BSR_ANY-
CRLF means that \R matches only CR, LF, or CRLF.
PCRE2_INFO_CAPTURECOUNT
Return the number of capturing subpatterns in the pattern. The third
argument should point to an uint32_t variable.
PCRE2_INFO_FIRSTCODETYPE
Return information about the first code unit of any matched string, for
a non-anchored pattern. The third argument should point to an uint32_t
variable.
If there is a fixed first value, for example, the letter "c" from a
pattern such as (cat|cow|coyote), 1 is returned, and the character
value can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no
fixed first value, but it is known that a match can occur only at the
start of the subject or following a newline in the subject, 2 is
returned. Otherwise, and for anchored patterns, 0 is returned.
PCRE2_INFO_FIRSTCODEUNIT
Return the value of the first code unit of any matched string in the
situation where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0.
The third argument should point to an uint32_t variable. In the 8-bit
library, the value is always less than 256. In the 16-bit library the
value can be up to 0xffff. In the 32-bit library in UTF-32 mode the
value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32
mode.
PCRE2_INFO_FIRSTBITMAP
In the absence of a single first code unit for a non-anchored pattern,
pcre2_compile() may construct a 256-bit table that defines a fixed set
of values for the first code unit in any match. For example, a pattern
that starts with [abc] results in a table with three bits set. When
code unit values greater than 255 are supported, the flag bit for 255
means "any code unit of value 255 or above". If such a table was con-
structed, a pointer to it is returned. Otherwise NULL is returned. The
third argument should point to an const uint8_t * variable.
PCRE2_INFO_HASCRORLF
Return 1 if the pattern contains any explicit matches for CR or LF
characters, otherwise 0. The third argument should point to an uint32_t
variable. An explicit match is either a literal CR or LF character, or
\r or \n.
PCRE2_INFO_JCHANGED
Return 1 if the (?J) or (?-J) option setting is used in the pattern,
otherwise 0. The third argument should point to an uint32_t variable.
(?J) and (?-J) set and unset the local PCRE2_DUPNAMES option, respec-
tively.
PCRE2_INFO_JITSIZE
If the compiled pattern was successfully processed by pcre2_jit_com-
pile(), return the size of the JIT compiled code, otherwise return
zero. The third argument should point to a size_t variable.
PCRE2_INFO_LASTCODETYPE
Returns 1 if there is a rightmost literal code unit that must exist in
any matched string, other than at its start. The third argument should
point to an uint32_t variable. If there is no such value, 0 is
returned. When 1 is returned, the code unit value itself can be
retrieved using PCRE2_INFO_LASTCODEUNIT.
For anchored patterns, a last literal value is recorded only if it fol-
lows something of variable length. For example, for the pattern
/^a\d+z\d+/ the returned value is 1 (with "z" returned from
PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value is 0.
PCRE2_INFO_LASTCODEUNIT
Return the value of the rightmost literal data unit that must exist in
any matched string, other than at its start, if such a value has been
recorded. The third argument should point to an uint32_t variable. If
there is no such value, 0 is returned.
PCRE2_INFO_MATCHEMPTY
Return 1 if the pattern can match an empty string, otherwise 0. The
third argument should point to an uint32_t variable.
PCRE2_INFO_MATCHLIMIT
If the pattern set a match limit by including an item of the form
(*LIMIT_MATCH=nnnn) at the start, the value is returned. The third
argument should point to an unsigned 32-bit integer. If no such value
has been set, the call to pcre2_pattern_info() returns the error
PCRE2_ERROR_UNSET.
PCRE2_INFO_MAXLOOKBEHIND
Return the number of characters (not code units) in the longest lookbe-
hind assertion in the pattern. The third argument should point to an
unsigned 32-bit integer. This information is useful when doing multi-
segment matching using the partial matching facilities. Note that the
simple assertions \b and \B require a one-character lookbehind. \A also
registers a one-character lookbehind, though it does not actually
inspect the previous character. This is to ensure that at least one
character from the old segment is retained when a new segment is pro-
cessed. Otherwise, if there are no lookbehinds in the pattern, \A might
match incorrectly at the start of a new segment.
PCRE2_INFO_MINLENGTH
If a minimum length for matching subject strings was computed, its
value is returned. Otherwise the returned value is 0. The value is a
number of characters, which in UTF mode may be different from the num-
ber of code units. The third argument should point to an uint32_t
variable. The value is a lower bound to the length of any matching
string. There may not be any strings of that length that do actually
match, but every string that does match is at least that long.
PCRE2_INFO_NAMECOUNT
PCRE2_INFO_NAMEENTRYSIZE
PCRE2_INFO_NAMETABLE
PCRE2 supports the use of named as well as numbered capturing parenthe-
ses. The names are just an additional way of identifying the parenthe-
ses, which still acquire numbers. Several convenience functions such as
pcre2_substring_get_byname() are provided for extracting captured sub-
strings by name. It is also possible to extract the data directly, by
first converting the name to a number in order to access the correct
pointers in the output vector (described with pcre2_match() below). To
do the conversion, you need to use the name-to-number map, which is
described by these three values.
The map consists of a number of fixed-size entries. PCRE2_INFO_NAME-
COUNT gives the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives
the size of each entry in code units; both of these return a uint32_t
value. The entry size depends on the length of the longest name.
PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table.
This is a PCRE2_SPTR pointer to a block of code units. In the 8-bit
library, the first two bytes of each entry are the number of the cap-
turing parenthesis, most significant byte first. In the 16-bit library,
the pointer points to 16-bit code units, the first of which contains
the parenthesis number. In the 32-bit library, the pointer points to
32-bit code units, the first of which contains the parenthesis number.
The rest of the entry is the corresponding name, zero terminated.
The names are in alphabetical order. If (?| is used to create multiple
groups with the same number, as described in the section on duplicate
subpattern numbers in the pcre2pattern page, the groups may be given
the same name, but there is only one entry in the table. Different
names for groups of the same number are not permitted.
Duplicate names for subpatterns with different numbers are permitted,
but only if PCRE2_DUPNAMES is set. They appear in the table in the
order in which they were found in the pattern. In the absence of (?|
this is the order of increasing number; when (?| is used this is not
necessarily the case because later subpatterns may have lower numbers.
As a simple example of the name/number table, consider the following
pattern after compilation by the 8-bit library (assume PCRE2_EXTENDED
is set, so white space - including newlines - is ignored):
(?<date> (?<year>(\d\d)?\d\d) -
(?<month>\d\d) - (?<day>\d\d) )
There are four named subpatterns, so the table has four entries, and
each entry in the table is eight bytes long. The table is as follows,
with non-printing bytes shows in hexadecimal, and undefined bytes shown
as ??:
00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??
When writing code to extract data from named subpatterns using the
name-to-number map, remember that the length of the entries is likely
to be different for each compiled pattern.
PCRE2_INFO_NEWLINE
The output is a uint32_t with one of the following values:
PCRE2_NEWLINE_CR Carriage return (CR)
PCRE2_NEWLINE_LF Linefeed (LF)
PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF)
PCRE2_NEWLINE_ANY Any Unicode line ending
PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF
This specifies the default character sequence that will be recognized
as meaning "newline" while matching.
PCRE2_INFO_RECURSIONLIMIT
If the pattern set a recursion limit by including an item of the form
(*LIMIT_RECURSION=nnnn) at the start, the value is returned. The third
argument should point to an unsigned 32-bit integer. If no such value
has been set, the call to pcre2_pattern_info() returns the error
PCRE2_ERROR_UNSET.
PCRE2_INFO_SIZE
Return the size of the compiled pattern in bytes (for all three
libraries). The third argument should point to a size_t variable. This
value includes the size of the general data block that precedes the
code units of the compiled pattern itself. The value that is used when
pcre2_compile() is getting memory in which to place the compiled pat-
tern may be slightly larger than the value returned by this option,
because there are cases where the code that calculates the size has to
over-estimate. Processing a pattern with the JIT compiler does not
alter the value returned by this option.
INFORMATION ABOUT A PATTERN'S CALLOUTS
int pcre2_callout_enumerate(const pcre2_code *code,
int (*callback)(pcre2_callout_enumerate_block *, void *),
void *user_data);
A script language that supports the use of string arguments in callouts
might like to scan all the callouts in a pattern before running the
match. This can be done by calling pcre2_callout_enumerate(). The first
argument is a pointer to a compiled pattern, the second points to a
callback function, and the third is arbitrary user data. The callback
function is called for every callout in the pattern in the order in
which they appear. Its first argument is a pointer to a callout enumer-
ation block, and its second argument is the user_data value that was
passed to pcre2_callout_enumerate(). The contents of the callout enu-
meration block are described in the pcre2callout documentation, which
also gives further details about callouts.
SERIALIZATION AND PRECOMPILING
It is possible to save compiled patterns on disc or elsewhere, and
reload them later, subject to a number of restrictions. The functions
whose names begin with pcre2_serialize_ are used for this purpose. They
are described in the pcre2serialize documentation.
THE MATCH DATA BLOCK
pcre2_match_data_create(uint32_t ovecsize,
pcre2_general_context *gcontext);
pcre2_match_data_create_from_pattern(const pcre2_code *code,
pcre2_general_context *gcontext);
void pcre2_match_data_free(pcre2_match_data *match_data);
Information about a successful or unsuccessful match is placed in a
match data block, which is an opaque structure that is accessed by
function calls. In particular, the match data block contains a vector
of offsets into the subject string that define the matched part of the
subject and any substrings that were captured. This is know as the
ovector.
Before calling pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()
you must create a match data block by calling one of the creation func-
tions above. For pcre2_match_data_create(), the first argument is the
number of pairs of offsets in the ovector. One pair of offsets is
required to identify the string that matched the whole pattern, with
another pair for each captured substring. For example, a value of 4
creates enough space to record the matched portion of the subject plus
three captured substrings. A minimum of at least 1 pair is imposed by
pcre2_match_data_create(), so it is always possible to return the over-
all matched string.
The second argument of pcre2_match_data_create() is a pointer to a gen-
eral context, which can specify custom memory management for obtaining
the memory for the match data block. If you are not using custom memory
management, pass NULL, which causes malloc() to be used.
For pcre2_match_data_create_from_pattern(), the first argument is a
pointer to a compiled pattern. The ovector is created to be exactly the
right size to hold all the substrings a pattern might capture. The sec-
ond argument is again a pointer to a general context, but in this case
if NULL is passed, the memory is obtained using the same allocator that
was used for the compiled pattern (custom or default).
A match data block can be used many times, with the same or different
compiled patterns. You can extract information from a match data block
after a match operation has finished, using functions that are
described in the sections on matched strings and other match data
below.
When a call of pcre2_match() fails, valid data is available in the
match block only when the error is PCRE2_ERROR_NOMATCH,
PCRE2_ERROR_PARTIAL, or one of the error codes for an invalid UTF
string. Exactly what is available depends on the error, and is detailed
below.
When one of the matching functions is called, pointers to the compiled
pattern and the subject string are set in the match data block so that
they can be referenced by the extraction functions. After running a
match, you must not free a compiled pattern or a subject string until
after all operations on the match data block (for that match) have
taken place.
When a match data block itself is no longer needed, it should be freed
by calling pcre2_match_data_free().
MATCHING A PATTERN: THE TRADITIONAL FUNCTION
int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext);
The function pcre2_match() is called to match a subject string against
a compiled pattern, which is passed in the code argument. You can call
pcre2_match() with the same code argument as many times as you like, in
order to find multiple matches in the subject string or to match dif-
ferent subject strings with the same pattern.
This function is the main matching facility of the library, and it
operates in a Perl-like manner. For specialist use there is also an
alternative matching function, which is described below in the section
about the pcre2_dfa_match() function.
Here is an example of a simple call to pcre2_match():
pcre2_match_data *md = pcre2_match_data_create(4, NULL);
int rc = pcre2_match(
re, /* result of pcre2_compile() */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
match_data, /* the match data block */
NULL); /* a match context; NULL means use defaults */
If the subject string is zero-terminated, the length can be given as
PCRE2_ZERO_TERMINATED. A match context must be provided if certain less
common matching parameters are to be changed. For details, see the sec-
tion on the match context above.
The string to be matched by pcre2_match()
The subject string is passed to pcre2_match() as a pointer in subject,
a length in length, and a starting offset in startoffset. The length
and offset are in code units, not characters. That is, they are in
bytes for the 8-bit library, 16-bit code units for the 16-bit library,
and 32-bit code units for the 32-bit library, whether or not UTF pro-
cessing is enabled.
If startoffset is greater than the length of the subject, pcre2_match()
returns PCRE2_ERROR_BADOFFSET. When the starting offset is zero, the
search for a match starts at the beginning of the subject, and this is
by far the most common case. In UTF-8 or UTF-16 mode, the starting off-
set must point to the start of a character, or to the end of the sub-
ject (in UTF-32 mode, one code unit equals one character, so all off-
sets are valid). Like the pattern string, the subject may contain
binary zeroes.
A non-zero starting offset is useful when searching for another match
in the same subject by calling pcre2_match() again after a previous
success. Setting startoffset differs from passing over a shortened
string and setting PCRE2_NOTBOL in the case of a pattern that begins
with any kind of lookbehind. For example, consider the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B matches
only if the current position in the subject is not a word boundary.)
When applied to the string "Mississipi" the first call to pcre2_match()
finds the first occurrence. If pcre2_match() is called again with just
the remainder of the subject, namely "issipi", it does not match,
because \B is always false at the start of the subject, which is deemed
to be a word boundary. However, if pcre2_match() is passed the entire
string again, but with startoffset set to 4, it finds the second occur-
rence of "iss" because it is able to look behind the starting point to
discover that it is preceded by a letter.
Finding all the matches in a subject is tricky when the pattern can
match an empty string. It is possible to emulate Perl's /g behaviour by
first trying the match again at the same offset, with the
PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED options, and then if that
fails, advancing the starting offset and trying an ordinary match
again. There is some code that demonstrates how to do this in the
pcre2demo sample program. In the most general case, you have to check
to see if the newline convention recognizes CRLF as a newline, and if
so, and the current character is CR followed by LF, advance the start-
ing offset by two characters instead of one.
If a non-zero starting offset is passed when the pattern is anchored,
one attempt to match at the given offset is made. This can only succeed
if the pattern does not require the match to be at the start of the
subject.
Option bits for pcre2_match()
The unused bits of the options argument for pcre2_match() must be zero.
The only bits that may be set are PCRE2_ANCHORED, PCRE2_NOTBOL,
PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. Their
action is described below.
Setting PCRE2_ANCHORED at match time is not supported by the just-in-
time (JIT) compiler. If it is set, JIT matching is disabled and the
normal interpretive code in pcre2_match() is run. The remaining options
are supported for JIT matching.
PCRE2_ANCHORED
The PCRE2_ANCHORED option limits pcre2_match() to matching at the first
matching position. If a pattern was compiled with PCRE2_ANCHORED, or
turned out to be anchored by virtue of its contents, it cannot be made
unachored at matching time. Note that setting the option at match time
disables JIT matching.
PCRE2_NOTBOL
This option specifies that first character of the subject string is not
the beginning of a line, so the circumflex metacharacter should not
match before it. Setting this without having set PCRE2_MULTILINE at
compile time causes circumflex never to match. This option affects only
the behaviour of the circumflex metacharacter. It does not affect \A.
PCRE2_NOTEOL
This option specifies that the end of the subject string is not the end
of a line, so the dollar metacharacter should not match it nor (except
in multiline mode) a newline immediately before it. Setting this with-
out having set PCRE2_MULTILINE at compile time causes dollar never to
match. This option affects only the behaviour of the dollar metacharac-
ter. It does not affect \Z or \z.
PCRE2_NOTEMPTY
An empty string is not considered to be a valid match if this option is
set. If there are alternatives in the pattern, they are tried. If all
the alternatives match the empty string, the entire match fails. For
example, if the pattern
a?b?
is applied to a string not beginning with "a" or "b", it matches an
empty string at the start of the subject. With PCRE2_NOTEMPTY set, this
match is not valid, so pcre2_match() searches further into the string
for occurrences of "a" or "b".
PCRE2_NOTEMPTY_ATSTART
This is like PCRE2_NOTEMPTY, except that it locks out an empty string
match only at the first matching position, that is, at the start of the
subject plus the starting offset. An empty string match later in the
subject is permitted. If the pattern is anchored, such a match can
occur only if the pattern contains \K.
PCRE2_NO_UTF_CHECK
When PCRE2_UTF is set at compile time, the validity of the subject as a
UTF string is checked by default when pcre2_match() is subsequently
called. The entire string is checked before any other processing takes
place, and a negative error code is returned if the check fails. There
are several UTF error codes for each code unit width, corresponding to
different problems with the code unit sequence. The value of startoff-
set is also checked, to ensure that it points to the start of a charac-
ter or to the end of the subject. There are discussions about the
validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the
pcre2unicode page.
If you know that your subject is valid, and you want to skip these
checks for performance reasons, you can set the PCRE2_NO_UTF_CHECK
option when calling pcre2_match(). You might want to do this for the
second and subsequent calls to pcre2_match() if you are making repeated
calls to find all the matches in a single subject string.
NOTE: When PCRE2_NO_UTF_CHECK is set, the effect of passing an invalid
string as a subject, or an invalid value of startoffset, is undefined.
Your program may crash or loop indefinitely.
PCRE2_PARTIAL_HARD
PCRE2_PARTIAL_SOFT
These options turn on the partial matching feature. A partial match
occurs if the end of the subject string is reached successfully, but
there are not enough subject characters to complete the match. If this
happens when PCRE2_PARTIAL_SOFT (but not PCRE2_PARTIAL_HARD) is set,
matching continues by testing any remaining alternatives. Only if no
complete match can be found is PCRE2_ERROR_PARTIAL returned instead of
PCRE2_ERROR_NOMATCH. In other words, PCRE2_PARTIAL_SOFT specifies that
the caller is prepared to handle a partial match, but only if no com-
plete match can be found.
If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this
case, if a partial match is found, pcre2_match() immediately returns
PCRE2_ERROR_PARTIAL, without considering any other alternatives. In
other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid-
ered to be more important that an alternative complete match.
There is a more detailed discussion of partial and multi-segment match-
ing, with examples, in the pcre2partial documentation.
NEWLINE HANDLING WHEN MATCHING
When PCRE2 is built, a default newline convention is set; this is usu-
ally the standard convention for the operating system. The default can
be overridden in a compile context. During matching, the newline
choice affects the behaviour of the dot, circumflex, and dollar
metacharacters. It may also alter the way the match starting position
is advanced after a match failure for an unanchored pattern.
When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is
set as the newline convention, and a match attempt for an unanchored
pattern fails when the current starting position is at a CRLF sequence,
and the pattern contains no explicit matches for CR or LF characters,
the match position is advanced by two characters instead of one, in
other words, to after the CRLF.
The above rule is a compromise that makes the most common cases work as
expected. For example, if the pattern is .+A (and the PCRE2_DOTALL
option is not set), it does not match the string "\r\nA" because, after
failing at the start, it skips both the CR and the LF before retrying.
However, the pattern [\r\n]A does match that string, because it con-
tains an explicit CR or LF reference, and so advances only by one char-
acter after the first failure.
An explicit match for CR of LF is either a literal appearance of one of
those characters in the pattern, or one of the \r or \n escape
sequences. Implicit matches such as [^X] do not count, nor does \s,
even though it includes CR and LF in the characters that it matches.
Notwithstanding the above, anomalous effects may still occur when CRLF
is a valid newline sequence and explicit \r or \n escapes appear in the
pattern.
HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS
uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);
PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);
In general, a pattern matches a certain portion of the subject, and in
addition, further substrings from the subject may be picked out by
parenthesized parts of the pattern. Following the usage in Jeffrey
Friedl's book, this is called "capturing" in what follows, and the
phrase "capturing subpattern" or "capturing group" is used for a frag-
ment of a pattern that picks out a substring. PCRE2 supports several
other kinds of parenthesized subpattern that do not cause substrings to
be captured. The pcre2_pattern_info() function can be used to find out
how many capturing subpatterns there are in a compiled pattern.
A successful match returns the overall matched string and any captured
substrings to the caller via a vector of PCRE2_SIZE values. This is
called the ovector, and is contained within the match data block. You
can obtain direct access to the ovector by calling pcre2_get_ovec-
tor_pointer() to find its address, and pcre2_get_ovector_count() to
find the number of pairs of values it contains. Alternatively, you can
use the auxiliary functions for accessing captured substrings by number
or by name (see below).
Within the ovector, the first in each pair of values is set to the off-
set of the first code unit of a substring, and the second is set to the
offset of the first code unit after the end of a substring. These val-
ues are always code unit offsets, not character offsets. That is, they
are byte offsets in the 8-bit library, 16-bit offsets in the 16-bit
library, and 32-bit offsets in the 32-bit library.
After a partial match (error return PCRE2_ERROR_PARTIAL), only the
first pair of offsets (that is, ovector[0] and ovector[1]) are set.
They identify the part of the subject that was partially matched. See
the pcre2partial documentation for details of partial matching.
After a successful match, the first pair of offsets identifies the por-
tion of the subject string that was matched by the entire pattern. The
next pair is used for the first capturing subpattern, and so on. The
value returned by pcre2_match() is one more than the highest numbered
pair that has been set. For example, if two substrings have been cap-
tured, the returned value is 3. If there are no capturing subpatterns,
the return value from a successful match is 1, indicating that just the
first pair of offsets has been set.
If a pattern uses the \K escape sequence within a positive assertion,
the reported start of a successful match can be greater than the end of
the match. For example, if the pattern (?=ab\K) is matched against
"ab", the start and end offset values for the match are 2 and 0.
If a capturing subpattern group is matched repeatedly within a single
match operation, it is the last portion of the subject that it matched
that is returned.
If the ovector is too small to hold all the captured substring offsets,
as much as possible is filled in, and the function returns a value of
zero. If captured substrings are not of interest, pcre2_match() may be
called with a match data block whose ovector is of minimum length (that
is, one pair). However, if the pattern contains back references and the
ovector is not big enough to remember the related substrings, PCRE2 has
to get additional memory for use during matching. Thus it is usually
advisable to set up a match data block containing an ovector of reason-
able size.
It is possible for capturing subpattern number n+1 to match some part
of the subject when subpattern n has not been used at all. For example,
if the string "abc" is matched against the pattern (a|(z))(bc) the
return from the function is 4, and subpatterns 1 and 3 are matched, but
2 is not. When this happens, both values in the offset pairs corre-
sponding to unused subpatterns are set to PCRE2_UNSET.
Offset values that correspond to unused subpatterns at the end of the
expression are also set to PCRE2_UNSET. For example, if the string
"abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3
are not matched. The return from the function is 2, because the high-
est used capturing subpattern number is 1. The offsets for for the sec-
ond and third capturing subpatterns (assuming the vector is large
enough, of course) are set to PCRE2_UNSET.
Elements in the ovector that do not correspond to capturing parentheses
in the pattern are never changed. That is, if a pattern contains n cap-
turing parentheses, no more than ovector[0] to ovector[2n+1] are set by
pcre2_match(). The other elements retain whatever values they previ-
ously had.
OTHER INFORMATION ABOUT A MATCH
PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);
As well as the offsets in the ovector, other information about a match
is retained in the match data block and can be retrieved by the above
functions in appropriate circumstances. If they are called at other
times, the result is undefined.
After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a
failure to match (PCRE2_ERROR_NOMATCH), a (*MARK) name may be avail-
able, and pcre2_get_mark() can be called. It returns a pointer to the
zero-terminated name, which is within the compiled pattern. Otherwise
NULL is returned. After a successful match, the (*MARK) name that is
returned is the last one encountered on the matching path through the
pattern. After a "no match" or a partial match, the last encountered
(*MARK) name is returned. For example, consider this pattern:
^(*MARK:A)((*MARK:B)a|b)c
When it matches "bc", the returned mark is A. The B mark is "seen" in
the first branch of the group, but it is not on the matching path. On
the other hand, when this pattern fails to match "bx", the returned
mark is B.
After a successful match, a partial match, or one of the invalid UTF
errors (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can
be called. After a successful or partial match it returns the code unit
offset of the character at which the match started. For a non-partial
match, this can be different to the value of ovector[0] if the pattern
contains the \K escape sequence. After a partial match, however, this
value is always the same as ovector[0] because \K does not affect the
result of a partial match.
After a UTF check failure, pcre2_get_startchar() can be used to obtain
the code unit offset of the invalid UTF character. Details are given in
the pcre2unicode page.
ERROR RETURNS FROM pcre2_match()
If pcre2_match() fails, it returns a negative number. This can be con-
verted to a text string by calling pcre2_get_error_message(). Negative
error codes are also returned by other functions, and are documented
with them. The codes are given names in the header file. If UTF check-
ing is in force and an invalid UTF subject string is detected, one of a
number of UTF-specific negative error codes is returned. Details are
given in the pcre2unicode page. The following are the other errors that
may be returned by pcre2_match():
PCRE2_ERROR_NOMATCH
The subject string did not match the pattern.
PCRE2_ERROR_PARTIAL
The subject string did not match, but it did match partially. See the
pcre2partial documentation for details of partial matching.
PCRE2_ERROR_BADMAGIC
PCRE2 stores a 4-byte "magic number" at the start of the compiled code,
to catch the case when it is passed a junk pointer. This is the error
that is returned when the magic number is not present.
PCRE2_ERROR_BADMODE
This error is given when a pattern that was compiled by the 8-bit
library is passed to a 16-bit or 32-bit library function, or vice
versa.
PCRE2_ERROR_BADOFFSET
The value of startoffset was greater than the length of the subject.
PCRE2_ERROR_BADOPTION
An unrecognized bit was set in the options argument.
PCRE2_ERROR_BADUTFOFFSET
The UTF code unit sequence that was passed as a subject was checked and
found to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the
value of startoffset did not point to the beginning of a UTF character
or the end of the subject.
PCRE2_ERROR_CALLOUT
This error is never generated by pcre2_match() itself. It is provided
for use by callout functions that want to cause pcre2_match() or
pcre2_callout_enumerate() to return a distinctive error code. See the
pcre2callout documentation for details.
PCRE2_ERROR_INTERNAL
An unexpected internal error has occurred. This error could be caused
by a bug in PCRE2 or by overwriting of the compiled pattern.
PCRE2_ERROR_JIT_BADOPTION
This error is returned when a pattern that was successfully studied
using JIT is being matched, but the matching mode (partial or complete
match) does not correspond to any JIT compilation mode. When the JIT
fast path function is used, this error may be also given for invalid
options. See the pcre2jit documentation for more details.
PCRE2_ERROR_JIT_STACKLIMIT
This error is returned when a pattern that was successfully studied
using JIT is being matched, but the memory available for the just-in-
time processing stack is not large enough. See the pcre2jit documenta-
tion for more details.
PCRE2_ERROR_MATCHLIMIT
The backtracking limit was reached.
PCRE2_ERROR_NOMEMORY
If a pattern contains back references, but the ovector is not big
enough to remember the referenced substrings, PCRE2 gets a block of
memory at the start of matching to use for this purpose. There are some
other special cases where extra memory is needed during matching. This
error is given when memory cannot be obtained.
PCRE2_ERROR_NULL
Either the code, subject, or match_data argument was passed as NULL.
PCRE2_ERROR_RECURSELOOP
This error is returned when pcre2_match() detects a recursion loop
within the pattern. Specifically, it means that either the whole pat-
tern or a subpattern has been called recursively for the second time at
the same position in the subject string. Some simple patterns that
might do this are detected and faulted at compile time, but more com-
plicated cases, in particular mutual recursions between two different
subpatterns, cannot be detected until matching is attempted.
PCRE2_ERROR_RECURSIONLIMIT
The internal recursion limit was reached.
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
uint32_t number, PCRE2_SIZE *length);
int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
uint32_t number, PCRE2_UCHAR *buffer,
PCRE2_SIZE *bufflen);
int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
uint32_t number, PCRE2_UCHAR **bufferptr,
PCRE2_SIZE *bufflen);
void pcre2_substring_free(PCRE2_UCHAR *buffer);
Captured substrings can be accessed directly by using the ovector as
described above. For convenience, auxiliary functions are provided for
extracting captured substrings as new, separate, zero-terminated
strings. A substring that contains a binary zero is correctly extracted
and has a further zero added on the end, but the result is not, of
course, a C string.
The functions in this section identify substrings by number. The number
zero refers to the entire matched substring, with higher numbers refer-
ring to substrings captured by parenthesized groups. After a partial
match, only substring zero is available. An attempt to extract any
other substring gives the error PCRE2_ERROR_PARTIAL. The next section
describes similar functions for extracting captured substrings by name.
If a pattern uses the \K escape sequence within a positive assertion,
the reported start of a successful match can be greater than the end of
the match. For example, if the pattern (?=ab\K) is matched against
"ab", the start and end offset values for the match are 2 and 0. In
this situation, calling these functions with a zero substring number
extracts a zero-length empty string.
You can find the length in code units of a captured substring without
extracting it by calling pcre2_substring_length_bynumber(). The first
argument is a pointer to the match data block, the second is the group
number, and the third is a pointer to a variable into which the length
is placed. If you just want to know whether or not the substring has
been captured, you can pass the third argument as NULL.
The pcre2_substring_copy_bynumber() function copies a captured sub-
string into a supplied buffer, whereas pcre2_substring_get_bynumber()
copies it into new memory, obtained using the same memory allocation
function that was used for the match data block. The first two argu-
ments of these functions are a pointer to the match data block and a
capturing group number.
The final arguments of pcre2_substring_copy_bynumber() are a pointer to
the buffer and a pointer to a variable that contains its length in code
units. This is updated to contain the actual number of code units used
for the extracted substring, excluding the terminating zero.
For pcre2_substring_get_bynumber() the third and fourth arguments point
to variables that are updated with a pointer to the new memory and the
number of code units that comprise the substring, again excluding the
terminating zero. When the substring is no longer needed, the memory
should be freed by calling pcre2_substring_free().
The return value from all these functions is zero for success, or a
negative error code. If the pattern match failed, the match failure
code is returned. If a substring number greater than zero is used
after a partial match, PCRE2_ERROR_PARTIAL is returned. Other possible
error codes are:
PCRE2_ERROR_NOMEMORY
The buffer was too small for pcre2_substring_copy_bynumber(), or the
attempt to get memory failed for pcre2_substring_get_bynumber().
PCRE2_ERROR_NOSUBSTRING
There is no substring with that number in the pattern, that is, the
number is greater than the number of capturing parentheses.
PCRE2_ERROR_UNAVAILABLE
The substring number, though not greater than the number of captures in
the pattern, is greater than the number of slots in the ovector, so the
substring could not be captured.
PCRE2_ERROR_UNSET
The substring did not participate in the match. For example, if the
pattern is (abc)|(def) and the subject is "def", and the ovector con-
tains at least two capturing slots, substring number 1 is unset.
EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS
int pcre2_substring_list_get(pcre2_match_data *match_data,
PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);
void pcre2_substring_list_free(PCRE2_SPTR *list);
The pcre2_substring_list_get() function extracts all available sub-
strings and builds a list of pointers to them. It also (optionally)
builds a second list that contains their lengths (in code units),
excluding a terminating zero that is added to each of them. All this is
done in a single block of memory that is obtained using the same memory
allocation function that was used to get the match data block.
This function must be called only after a successful match. If called
after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.
The address of the memory block is returned via listptr, which is also
the start of the list of string pointers. The end of the list is marked
by a NULL pointer. The address of the list of lengths is returned via
lengthsptr. If your strings do not contain binary zeros and you do not
therefore need the lengths, you may supply NULL as the lengthsptr argu-
ment to disable the creation of a list of lengths. The yield of the
function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the mem-
ory block could not be obtained. When the list is no longer needed, it
should be freed by calling pcre2_substring_list_free().
If this function encounters a substring that is unset, which can happen
when capturing subpattern number n+1 matches some part of the subject,
but subpattern n has not been used at all, it returns an empty string.
This can be distinguished from a genuine zero-length substring by
inspecting the appropriate offset in the ovector, which contain
PCRE2_UNSET for unset substrings, or by calling pcre2_sub-
string_length_bynumber().
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre2_substring_number_from_name(const pcre2_code *code,
PCRE2_SPTR name);
int pcre2_substring_length_byname(pcre2_match_data *match_data,
PCRE2_SPTR name, PCRE2_SIZE *length);
int pcre2_substring_copy_byname(pcre2_match_data *match_data,
PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);
int pcre2_substring_get_byname(pcre2_match_data *match_data,
PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);
void pcre2_substring_free(PCRE2_UCHAR *buffer);
To extract a substring by name, you first have to find associated num-
ber. For example, for this pattern:
(a+)b(?<xxx>\d+)...
the number of the subpattern called "xxx" is 2. If the name is known to
be unique (PCRE2_DUPNAMES was not set), you can find the number from
the name by calling pcre2_substring_number_from_name(). The first argu-
ment is the compiled pattern, and the second is the name. The yield of
the function is the subpattern number, PCRE2_ERROR_NOSUBSTRING if there
is no subpattern of that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if
there is more than one subpattern of that name. Given the number, you
can extract the substring directly, or use one of the functions
described above.
For convenience, there are also "byname" functions that correspond to
the "bynumber" functions, the only difference being that the second
argument is a name instead of a number. If PCRE2_DUPNAMES is set and
there are duplicate names, these functions scan all the groups with the
given name, and return the first named string that is set.
If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is
returned. If all groups with the name have numbers that are greater
than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is
returned. If there is at least one group with a slot in the ovector,
but no group is found to be set, PCRE2_ERROR_UNSET is returned.
Warning: If the pattern uses the (?| feature to set up multiple subpat-
terns with the same number, as described in the section on duplicate
subpattern numbers in the pcre2pattern page, you cannot use names to
distinguish the different subpatterns, because names are not included
in the compiled code. The matching process uses only numbers. For this
reason, the use of different names for subpatterns of the same number
causes an error at compile time.
CREATING A NEW STRING WITH SUBSTITUTIONS
int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext, PCRE2_SPTR replacementzfP,
PCRE2_SIZE rlength, PCRE2_UCHAR *outputbufferP,
PCRE2_SIZE *outlengthptr);
This function calls pcre2_match() and then makes a copy of the subject
string in outputbuffer, replacing the part that was matched with the
replacement string, whose length is supplied in rlength. This can be
given as PCRE2_ZERO_TERMINATED for a zero-terminated string.
In the replacement string, which is interpreted as a UTF string in UTF
mode, and is checked for UTF validity unless the PCRE2_NO_UTF_CHECK
option is set, a dollar character is an escape character that can spec-
ify the insertion of characters from capturing groups in the pattern.
The following forms are recognized:
$$ insert a dollar character
$<n> insert the contents of group <n>
${<n>} insert the contents of group <n>
Either a group number or a group name can be given for <n>. Curly
brackets are required only if the following character would be inter-
preted as part of the number or name. The number may be zero to include
the entire matched string. For example, if the pattern a(b)c is
matched with "=abc=" and the replacement string "+$1$0$1+", the result
is "=+babcb+=". Group insertion is done by calling pcre2_copy_byname()
or pcre2_copy_bynumber() as appropriate.
The first seven arguments of pcre2_substitute() are the same as for
pcre2_match(), except that the partial matching options are not permit-
ted, and match_data may be passed as NULL, in which case a match data
block is obtained and freed within this function, using memory manage-
ment functions from the match context, if provided, or else those that
were used to allocate memory for the compiled code.
There is one additional option, PCRE2_SUBSTITUTE_GLOBAL, which causes
the function to iterate over the subject string, replacing every match-
ing substring. If this is not set, only the first matching substring is
replaced.
The outlengthptr argument must point to a variable that contains the
length, in code units, of the output buffer. It is updated to contain
the length of the new string, excluding the trailing zero that is auto-
matically added.
The function returns the number of replacements that were made. This
may be zero if no matches were found, and is never greater than 1
unless PCRE2_SUBSTITUTE_GLOBAL is set. In the event of an error, a neg-
ative error code is returned. Except for PCRE2_ERROR_NOMATCH (which is
never returned), any errors from pcre2_match() or the substring copying
functions are passed straight back. PCRE2_ERROR_BADREPLACEMENT is
returned for an invalid replacement string (unrecognized sequence fol-
lowing a dollar sign), and PCRE2_ERROR_NOMEMORY is returned if the out-
put buffer is not big enough.
DUPLICATE SUBPATTERN NAMES
int pcre2_substring_nametable_scan(const pcre2_code *code,
PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);
When a pattern is compiled with the PCRE2_DUPNAMES option, names for
subpatterns are not required to be unique. Duplicate names are always
allowed for subpatterns with the same number, created by using the (?|
feature. Indeed, if such subpatterns are named, they are required to
use the same names.
Normally, patterns with duplicate names are such that in any one match,
only one of the named subpatterns participates. An example is shown in
the pcre2pattern documentation.
When duplicates are present, pcre2_substring_copy_byname() and
pcre2_substring_get_byname() return the first substring corresponding
to the given name that is set. Only if none are set is
PCRE2_ERROR_UNSET is returned. The pcre2_substring_number_from_name()
function returns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are
duplicate names.
If you want to get full details of all captured substrings for a given
name, you must use the pcre2_substring_nametable_scan() function. The
first argument is the compiled pattern, and the second is the name. If
the third and fourth arguments are NULL, the function returns a group
number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.
When the third and fourth arguments are not NULL, they must be pointers
to variables that are updated by the function. After it has run, they
point to the first and last entries in the name-to-number table for the
given name, and the function returns the length of each entry in code
units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there are
no entries for the given name.
The format of the name table is described above in the section entitled
Information about a pattern above. Given all the relevant entries for
the name, you can extract each of their numbers, and hence the captured
data.
FINDING ALL POSSIBLE MATCHES AT ONE POSITION
The traditional matching function uses a similar algorithm to Perl,
which stops when it finds the first match at a given point in the sub-
ject. If you want to find all possible matches, or the longest possible
match at a given position, consider using the alternative matching
function (see below) instead. If you cannot use the alternative func-
tion, you can kludge it up by making use of the callout facility, which
is described in the pcre2callout documentation.
What you have to do is to insert a callout right at the end of the pat-
tern. When your callout function is called, extract and save the cur-
rent matched substring. Then return 1, which forces pcre2_match() to
backtrack and try other alternatives. Ultimately, when it runs out of
matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.
MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
PCRE2_SIZE length, PCRE2_SIZE startoffset,
uint32_t options, pcre2_match_data *match_data,
pcre2_match_context *mcontext,
int *workspace, PCRE2_SIZE wscount);
The function pcre2_dfa_match() is called to match a subject string
against a compiled pattern, using a matching algorithm that scans the
subject string just once, and does not backtrack. This has different
characteristics to the normal algorithm, and is not compatible with
Perl. Some of the features of PCRE2 patterns are not supported. Never-
theless, there are times when this kind of matching can be useful. For
a discussion of the two matching algorithms, and a list of features
that pcre2_dfa_match() does not support, see the pcre2matching documen-
tation.
The arguments for the pcre2_dfa_match() function are the same as for
pcre2_match(), plus two extras. The ovector within the match data block
is used in a different way, and this is described below. The other com-
mon arguments are used in the same way as for pcre2_match(), so their
description is not repeated here.
The two additional arguments provide workspace for the function. The
workspace vector should contain at least 20 elements. It is used for
keeping track of multiple paths through the pattern tree. More
workspace is needed for patterns and subjects where there are a lot of
potential matches.
Here is an example of a simple call to pcre2_dfa_match():
int wspace[20];
pcre2_match_data *md = pcre2_match_data_create(4, NULL);
int rc = pcre2_dfa_match(
re, /* result of pcre2_compile() */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
match_data, /* the match data block */
NULL, /* a match context; NULL means use defaults */
wspace, /* working space vector */
20); /* number of elements (NOT size in bytes) */
Option bits for pcre_dfa_match()
The unused bits of the options argument for pcre2_dfa_match() must be
zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_NOTBOL,
PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT,
PCRE2_DFA_SHORTEST, and PCRE2_DFA_RESTART. All but the last four of
these are exactly the same as for pcre2_match(), so their description
is not repeated here.
PCRE2_PARTIAL_HARD
PCRE2_PARTIAL_SOFT
These have the same general effect as they do for pcre2_match(), but
the details are slightly different. When PCRE2_PARTIAL_HARD is set for
pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if the end of the
subject is reached and there is still at least one matching possibility
that requires additional characters. This happens even if some complete
matches have already been found. When PCRE2_PARTIAL_SOFT is set, the
return code PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL
if the end of the subject is reached, there have been no complete
matches, but there is still at least one matching possibility. The por-
tion of the string that was inspected when the longest partial match
was found is set as the first matching string in both cases. There is a
more detailed discussion of partial and multi-segment matching, with
examples, in the pcre2partial documentation.
PCRE2_DFA_SHORTEST
Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm to
stop as soon as it has found one match. Because of the way the alterna-
tive algorithm works, this is necessarily the shortest possible match
at the first possible matching point in the subject string.
PCRE2_DFA_RESTART
When pcre2_dfa_match() returns a partial match, it is possible to call
it again, with additional subject characters, and have it continue with
the same match. The PCRE2_DFA_RESTART option requests this action; when
it is set, the workspace and wscount options must reference the same
vector as before because data about the match so far is left in them
after a partial match. There is more discussion of this facility in the
pcre2partial documentation.
Successful returns from pcre2_dfa_match()
When pcre2_dfa_match() succeeds, it may have matched more than one sub-
string in the subject. Note, however, that all the matches from one run
of the function start at the same point in the subject. The shorter
matches are all initial substrings of the longer matches. For example,
if the pattern
<.*>
is matched against the string
This is <something> <something else> <something further> no more
the three matched strings are
<something> <something else> <something further>
<something> <something else>
<something>
On success, the yield of the function is a number greater than zero,
which is the number of matched substrings. The offsets of the sub-
strings are returned in the ovector, and can be extracted by number in
the same way as for pcre2_match(), but the numbers bear no relation to
any capturing groups that may exist in the pattern, because DFA match-
ing does not support group capture.
Calls to the convenience functions that extract substrings by name
return the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used
after a DFA match. The convenience functions that extract substrings by
number never return PCRE2_ERROR_NOSUBSTRING, and the meanings of some
other errors are slightly different:
PCRE2_ERROR_UNAVAILABLE
The ovector is not big enough to include a slot for the given substring
number.
PCRE2_ERROR_UNSET
There is a slot in the ovector for this substring, but there were
insufficient matches to fill it.
The matched strings are stored in the ovector in reverse order of
length; that is, the longest matching string is first. If there were
too many matches to fit into the ovector, the yield of the function is
zero, and the vector is filled with the longest matches.
NOTE: PCRE2's "auto-possessification" optimization usually applies to
character repeats at the end of a pattern (as well as internally). For
example, the pattern "a\d+" is compiled as if it were "a\d++". For DFA
matching, this means that only one possible match is found. If you
really do want multiple matches in such cases, either use an ungreedy
repeat auch as "a\d+?" or set the PCRE2_NO_AUTO_POSSESS option when
compiling.
Error returns from pcre2_dfa_match()
The pcre2_dfa_match() function returns a negative number when it fails.
Many of the errors are the same as for pcre2_match(), as described
above. There are in addition the following errors that are specific to
pcre2_dfa_match():
PCRE2_ERROR_DFA_UITEM
This return is given if pcre2_dfa_match() encounters an item in the
pattern that it does not support, for instance, the use of \C or a back
reference.
PCRE2_ERROR_DFA_UCOND
This return is given if pcre2_dfa_match() encounters a condition item
that uses a back reference for the condition, or a test for recursion
in a specific group. These are not supported.
PCRE2_ERROR_DFA_WSSIZE
This return is given if pcre2_dfa_match() runs out of space in the
workspace vector.
PCRE2_ERROR_DFA_RECURSE
When a recursive subpattern is processed, the matching function calls
itself recursively, using private memory for the ovector and workspace.
This error is given if the internal ovector is not large enough. This
should be extremely rare, as a vector of size 1000 is used.
PCRE2_ERROR_DFA_BADRESTART
When pcre2_dfa_match() is called with the PCRE2_DFA_RESTART option,
some plausibility checks are made on the contents of the workspace,
which should contain data about the previous partial match. If any of
these checks fail, this error is given.
SEE ALSO
pcre2build(3), pcre2callout(3), pcre2demo(3), pcre2matching(3),
pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2stack(3),
pcre2unicode(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 22 April 2015
Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------
PCRE2BUILD(3) Library Functions Manual PCRE2BUILD(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
BUILDING PCRE2
PCRE2 is distributed with a configure script that can be used to build
the library in Unix-like environments using the applications known as
Autotools. Also in the distribution are files to support building using
CMake instead of configure. The text file README contains general
information about building with Autotools (some of which is repeated
below), and also has some comments about building on various operating
systems. There is a lot more information about building PCRE2 without
using Autotools (including information about using CMake and building
"by hand") in the text file called NON-AUTOTOOLS-BUILD. You should
consult this file as well as the README file if you are building in a
non-Unix-like environment.
PCRE2 BUILD-TIME OPTIONS
The rest of this document describes the optional features of PCRE2 that
can be selected when the library is compiled. It assumes use of the
configure script, where the optional features are selected or dese-
lected by providing options to configure before running the make com-
mand. However, the same options can be selected in both Unix-like and
non-Unix-like environments if you are using CMake instead of configure
to build PCRE2.
If you are not using Autotools or CMake, option selection can be done
by editing the config.h file, or by passing parameter settings to the
compiler, as described in NON-AUTOTOOLS-BUILD.
The complete list of options for configure (which includes the standard
ones such as the selection of the installation directory) can be
obtained by running
./configure --help
The following sections include descriptions of options whose names
begin with --enable or --disable. These settings specify changes to the
defaults for the configure command. Because of the way that configure
works, --enable and --disable always come in pairs, so the complemen-
tary option always exists as well, but as it specifies the default, it
is not described.
BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES
By default, a library called libpcre2-8 is built, containing functions
that take string arguments contained in vectors of bytes, interpreted
either as single-byte characters, or UTF-8 strings. You can also build
two other libraries, called libpcre2-16 and libpcre2-32, which process
strings that are contained in vectors of 16-bit and 32-bit code units,
respectively. These can be interpreted either as single-unit characters
or UTF-16/UTF-32 strings. To build these additional libraries, add one
or both of the following to the configure command:
--enable-pcre2-16
--enable-pcre2-32
If you do not want the 8-bit library, add
--disable-pcre2-8
as well. At least one of the three libraries must be built. Note that
the POSIX wrapper is for the 8-bit library only, and that pcre2grep is
an 8-bit program. Neither of these are built if you select only the
16-bit or 32-bit libraries.
BUILDING SHARED AND STATIC LIBRARIES
The Autotools PCRE2 building process uses libtool to build both shared
and static libraries by default. You can suppress an unwanted library
by adding one of
--disable-shared
--disable-static
to the configure command.
UNICODE AND UTF SUPPORT
By default, PCRE2 is built with support for Unicode and UTF character
strings. To build it without Unicode support, add
--disable-unicode
to the configure command. This setting applies to all three libraries.
It is not possible to build one library with Unicode support, and
another without, in the same configuration.
Of itself, Unicode support does not make PCRE2 treat strings as UTF-8,
UTF-16 or UTF-32. To do that, applications that use the library can set
the PCRE2_UTF option when they call pcre2_compile() to compile a pat-
tern. Alternatively, patterns may be started with (*UTF) unless the
application has locked this out by setting PCRE2_NEVER_UTF.
UTF support allows the libraries to process character code points up to
0x10ffff in the strings that they handle. It also provides support for
accessing the Unicode properties of such characters, using pattern
escapes such as \P, \p, and \X. Only the general category properties
such as Lu and Nd are supported. Details are given in the pcre2pattern
documentation.
Pattern escapes such as \d and \w do not by default make use of Unicode
properties. The application can request that they do by setting the
PCRE2_UCP option. Unless the application has set PCRE2_NEVER_UCP, a
pattern may also request this by starting with (*UCP).
The \C escape sequence, which matches a single code unit, even in a UTF
mode, can cause unpredictable behaviour because it may leave the cur-
rent matching point in the middle of a multi-code-unit character. It
can be locked out by setting the PCRE2_NEVER_BACKSLASH_C option.
JUST-IN-TIME COMPILER SUPPORT
Just-in-time compiler support is included in the build by specifying
--enable-jit
This support is available only for certain hardware architectures. If
this option is set for an unsupported architecture, a building error
occurs. See the pcre2jit documentation for a discussion of JIT usage.
When JIT support is enabled, pcre2grep automatically makes use of it,
unless you add
--disable-pcre2grep-jit
to the "configure" command.
NEWLINE RECOGNITION
By default, PCRE2 interprets the linefeed (LF) character as indicating
the end of a line. This is the normal newline character on Unix-like
systems. You can compile PCRE2 to use carriage return (CR) instead, by
adding
--enable-newline-is-cr
to the configure command. There is also an --enable-newline-is-lf
option, which explicitly specifies linefeed as the newline character.
Alternatively, you can specify that line endings are to be indicated by
the two-character sequence CRLF (CR immediately followed by LF). If you
want this, add
--enable-newline-is-crlf
to the configure command. There is a fourth option, specified by
--enable-newline-is-anycrlf
which causes PCRE2 to recognize any of the three sequences CR, LF, or
CRLF as indicating a line ending. Finally, a fifth option, specified by
--enable-newline-is-any
causes PCRE2 to recognize any Unicode newline sequence. The Unicode
newline sequences are the three just mentioned, plus the single charac-
ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
U+0085), LS (line separator, U+2028), and PS (paragraph separator,
U+2029).
Whatever default line ending convention is selected when PCRE2 is built
can be overridden by applications that use the library. At build time
it is conventional to use the standard for your operating system.
WHAT \R MATCHES
By default, the sequence \R in a pattern matches any Unicode newline
sequence, independently of what has been selected as the line ending
sequence. If you specify
--enable-bsr-anycrlf
the default is changed so that \R matches only CR, LF, or CRLF. What-
ever is selected when PCRE2 is built can be overridden by applications
that use the called.
HANDLING VERY LARGE PATTERNS
Within a compiled pattern, offset values are used to point from one
part to another (for example, from an opening parenthesis to an alter-
nation metacharacter). By default, in the 8-bit and 16-bit libraries,
two-byte values are used for these offsets, leading to a maximum size
for a compiled pattern of around 64K code units. This is sufficient to
handle all but the most gigantic patterns. Nevertheless, some people do
want to process truly enormous patterns, so it is possible to compile
PCRE2 to use three-byte or four-byte offsets by adding a setting such
as
--with-link-size=3
to the configure command. The value given must be 2, 3, or 4. For the
16-bit library, a value of 3 is rounded up to 4. In these libraries,
using longer offsets slows down the operation of PCRE2 because it has
to load additional data when handling them. For the 32-bit library the
value is always 4 and cannot be overridden; the value of --with-link-
size is ignored.
AVOIDING EXCESSIVE STACK USAGE
When matching with the pcre2_match() function, PCRE2 implements back-
tracking by making recursive calls to an internal function called
match(). In environments where the size of the stack is limited, this
can severely limit PCRE2's operation. (The Unix environment does not
usually suffer from this problem, but it may sometimes be necessary to
increase the maximum stack size. There is a discussion in the
pcre2stack documentation.) An alternative approach to recursion that
uses memory from the heap to remember data, instead of using recursive
function calls, has been implemented to work round the problem of lim-
ited stack size. If you want to build a version of PCRE2 that works
this way, add
--disable-stack-for-recursion
to the configure command. By default, the system functions malloc() and
free() are called to manage the heap memory that is required, but cus-
tom memory management functions can be called instead. PCRE2 runs
noticeably more slowly when built in this way. This option affects only
the pcre2_match() function; it is not relevant for pcre2_dfa_match().
LIMITING PCRE2 RESOURCE USAGE
Internally, PCRE2 has a function called match(), which it calls repeat-
edly (sometimes recursively) when matching a pattern with the
pcre2_match() function. By controlling the maximum number of times this
function may be called during a single matching operation, a limit can
be placed on the resources used by a single call to pcre2_match(). The
limit can be changed at run time, as described in the pcre2api documen-
tation. The default is 10 million, but this can be changed by adding a
setting such as
--with-match-limit=500000
to the configure command. This setting has no effect on the
pcre2_dfa_match() matching function.
In some environments it is desirable to limit the depth of recursive
calls of match() more strictly than the total number of calls, in order
to restrict the maximum amount of stack (or heap, if --disable-stack-
for-recursion is specified) that is used. A second limit controls this;
it defaults to the value that is set for --with-match-limit, which
imposes no additional constraints. However, you can set a lower limit
by adding, for example,
--with-match-limit-recursion=10000
to the configure command. This value can also be overridden at run
time.
CREATING CHARACTER TABLES AT BUILD TIME
PCRE2 uses fixed tables for processing characters whose code points are
less than 256. By default, PCRE2 is built with a set of tables that are
distributed in the file src/pcre2_chartables.c.dist. These tables are
for ASCII codes only. If you add
--enable-rebuild-chartables
to the configure command, the distributed tables are no longer used.
Instead, a program called dftables is compiled and run. This outputs
the source for new set of tables, created in the default locale of your
C run-time system. (This method of replacing the tables does not work
if you are cross compiling, because dftables is run on the local host.
If you need to create alternative tables when cross compiling, you will
have to do so "by hand".)
USING EBCDIC CODE
PCRE2 assumes by default that it will run in an environment where the
character code is ASCII or Unicode, which is a superset of ASCII. This
is the case for most computer operating systems. PCRE2 can, however, be
compiled to run in an 8-bit EBCDIC environment by adding
--enable-ebcdic --disable-unicode
to the configure command. This setting implies --enable-rebuild-charta-
bles. You should only use it if you know that you are in an EBCDIC
environment (for example, an IBM mainframe operating system).
It is not possible to support both EBCDIC and UTF-8 codes in the same
version of the library. Consequently, --enable-unicode and --enable-
ebcdic are mutually exclusive.
The EBCDIC character that corresponds to an ASCII LF is assumed to have
the value 0x15 by default. However, in some EBCDIC environments, 0x25
is used. In such an environment you should use
--enable-ebcdic-nl25
as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR
has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and
0x25 is not chosen as LF is made to correspond to the Unicode NEL char-
acter (which, in Unicode, is 0x85).
The options that select newline behaviour, such as --enable-newline-is-
cr, and equivalent run-time options, refer to these character values in
an EBCDIC environment.
PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT
By default, pcre2grep reads all files as plain text. You can build it
so that it recognizes files whose names end in .gz or .bz2, and reads
them with libz or libbz2, respectively, by adding one or both of
--enable-pcre2grep-libz
--enable-pcre2grep-libbz2
to the configure command. These options naturally require that the rel-
evant libraries are installed on your system. Configuration will fail
if they are not.
PCRE2GREP BUFFER SIZE
pcre2grep uses an internal buffer to hold a "window" on the file it is
scanning, in order to be able to output "before" and "after" lines when
it finds a match. The size of the buffer is controlled by a parameter
whose default value is 20K. The buffer itself is three times this size,
but because of the way it is used for holding "before" lines, the long-
est line that is guaranteed to be processable is the parameter size.
You can change the default parameter value by adding, for example,
--with-pcre2grep-bufsize=50K
to the configure command. The caller of pcre2grep can override this
value by using --buffer-size on the command line..
PCRE2TEST OPTION FOR LIBREADLINE SUPPORT
If you add one of
--enable-pcre2test-libreadline
--enable-pcre2test-libedit
to the configure command, pcre2test is linked with the libreadline
orlibedit library, respectively, and when its input is from a terminal,
it reads it using the readline() function. This provides line-editing
and history facilities. Note that libreadline is GPL-licensed, so if
you distribute a binary of pcre2test linked in this way, there may be
licensing issues. These can be avoided by linking instead with libedit,
which has a BSD licence.
Setting --enable-pcre2test-libreadline causes the -lreadline option to
be added to the pcre2test build. In many operating environments with a
sytem-installed readline library this is sufficient. However, in some
environments (e.g. if an unmodified distribution version of readline is
in use), some extra configuration may be necessary. The INSTALL file
for libreadline says this:
"Readline uses the termcap functions, but does not link with
the termcap or curses library itself, allowing applications
which link with readline the to choose an appropriate library."
If your environment has not been set up so that an appropriate library
is automatically included, you may need to add something like
LIBS="-ncurses"
immediately before the configure command.
INCLUDING DEBUGGING CODE
If you add
--enable-debug
to the configure command, additional debugging code is included in the
build. This feature is intended for use by the PCRE2 maintainers.
DEBUGGING WITH VALGRIND SUPPORT
If you add
--enable-valgrind
to the configure command, PCRE2 will use valgrind annotations to mark
certain memory regions as unaddressable. This allows it to detect
invalid memory accesses, and is mostly useful for debugging PCRE2
itself.
CODE COVERAGE REPORTING
If your C compiler is gcc, you can build a version of PCRE2 that can
generate a code coverage report for its test suite. To enable this, you
must install lcov version 1.6 or above. Then specify
--enable-coverage
to the configure command and build PCRE2 in the usual way.
Note that using ccache (a caching C compiler) is incompatible with code
coverage reporting. If you have configured ccache to run automatically
on your system, you must set the environment variable
CCACHE_DISABLE=1
before running make to build PCRE2, so that ccache is not used.
When --enable-coverage is used, the following addition targets are
added to the Makefile:
make coverage
This creates a fresh coverage report for the PCRE2 test suite. It is
equivalent to running "make coverage-reset", "make coverage-baseline",
"make check", and then "make coverage-report".
make coverage-reset
This zeroes the coverage counters, but does nothing else.
make coverage-baseline
This captures baseline coverage information.
make coverage-report
This creates the coverage report.
make coverage-clean-report
This removes the generated coverage report without cleaning the cover-
age data itself.
make coverage-clean-data
This removes the captured coverage data without removing the coverage
files created at compile time (*.gcno).
make coverage-clean
This cleans all coverage data including the generated coverage report.
For more information about code coverage, see the gcov and lcov docu-
mentation.
SEE ALSO
pcre2api(3), pcre2-config(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 24 April 2015
Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------
PCRE2CALLOUT(3) Library Functions Manual PCRE2CALLOUT(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#include <pcre2.h>
int (*pcre2_callout)(pcre2_callout_block *, void *);
int pcre2_callout_enumerate(const pcre2_code *code,
int (*callback)(pcre2_callout_enumerate_block *, void *),
void *user_data);
DESCRIPTION
PCRE2 provides a feature called "callout", which is a means of tempo-
rarily passing control to the caller of PCRE2 in the middle of pattern
matching. The caller of PCRE2 provides an external function by putting
its entry point in a match context (see pcre2_set_callout() in the
pcre2api documentation).
Within a regular expression, (?C<arg>) indicates a point at which the
external function is to be called. Different callout points can be
identified by putting a number less than 256 after the letter C. The
default value is zero. Alternatively, the argument may be a delimited
string. The starting delimiter must be one of ` ' " ^ % # $ { and the
ending delimiter is the same as the start, except for {, where the end-
ing delimiter is }. If the ending delimiter is needed within the
string, it must be doubled. For example, this pattern has two callout
points:
(?C1)abc(?C"some ""arbitrary"" text")def
If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled,
PCRE2 automatically inserts callouts, all with number 255, before each
item in the pattern. For example, if PCRE2_AUTO_CALLOUT is used with
the pattern
A(\d{2}|--)
it is processed as if it were
(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
Notice that there is a callout before and after each parenthesis and
alternation bar. If the pattern contains a conditional group whose con-
dition is an assertion, an automatic callout is inserted immediately
before the condition. Such a callout may also be inserted explicitly,
for example:
(?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de)
This applies only to assertion conditions (because they are themselves
independent groups).
Callouts can be useful for tracking the progress of pattern matching.
The pcre2test program has a pattern qualifier (/auto_callout) that sets
automatic callouts. When any callouts are present, the output from
pcre2test indicates how the pattern is being matched. This is useful
information when you are trying to optimize the performance of a par-
ticular pattern.
MISSING CALLOUTS
You should be aware that, because of optimizations in the way PCRE2
compiles and matches patterns, callouts sometimes do not happen exactly
as you might expect.
Auto-possessification
At compile time, PCRE2 "auto-possessifies" repeated items when it knows
that what follows cannot be part of the repeat. For example, a+[bc] is
compiled as if it were a++[bc]. The pcre2test output when this pattern
is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
to the string "aaaa" is:
--->aaaa
+0 ^ a+
+2 ^ ^ [bc]
No match
This indicates that when matching [bc] fails, there is no backtracking
into a+ and therefore the callouts that would be taken for the back-
tracks do not occur. You can disable the auto-possessify feature by
passing PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pat-
tern with (*NO_AUTO_POSSESS). In this case, the output changes to this:
--->aaaa
+0 ^ a+
+2 ^ ^ [bc]
+2 ^ ^ [bc]
+2 ^ ^ [bc]
+2 ^^ [bc]
No match
This time, when matching [bc] fails, the matcher backtracks into a+ and
tries again, repeatedly, until a+ itself fails.
Automatic .* anchoring
By default, an optimization is applied when .* is the first significant
item in a pattern. If PCRE2_DOTALL is set, so that the dot can match
any character, the pattern is automatically anchored. If PCRE2_DOTALL
is not set, a match can start only after an internal newline or at the
beginning of the subject, and pcre2_compile() remembers this. This
optimization is disabled, however, if .* is in an atomic group or if
there is a back reference to the capturing group in which it appears.
It is also disabled if the pattern contains (*PRUNE) or (*SKIP). How-
ever, the presence of callouts does not affect it.
For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT
and applied to the string "aa", the pcre2test output is:
--->aa
+0 ^ .*
+2 ^ ^ \d
+2 ^^ \d
+2 ^ \d
No match
This shows that all match attempts start at the beginning of the sub-
ject. In other words, the pattern is anchored. You can disable this
optimization by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or
starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the out-
put changes to:
--->aa
+0 ^ .*
+2 ^ ^ \d
+2 ^^ \d
+2 ^ \d
+0 ^ .*
+2 ^^ \d
+2 ^ \d
No match
This shows more match attempts, starting at the second subject charac-
ter. Another optimization, described in the next section, means that
there is no subsequent attempt to match with an empty subject.
If a pattern has more than one top-level branch, automatic anchoring
occurs if all branches are anchorable.
Other optimizations
Other optimizations that provide fast "no match" results also affect
callouts. For example, if the pattern is
ab(?C4)cd
PCRE2 knows that any matching string must contain the letter "d". If
the subject string is "abyz", the lack of "d" means that matching
doesn't ever start, and the callout is never reached. However, with
"abyd", though the result is still no match, the callout is obeyed.
PCRE2 also knows the minimum length of a matching string, and will
immediately give a "no match" return without actually running a match
if the subject is not long enough, or, for unanchored patterns, if it
has been scanned far enough.
You can disable these optimizations by passing the PCRE2_NO_START_OPTI-
MIZE option to pcre2_compile(), or by starting the pattern with
(*NO_START_OPT). This slows down the matching process, but does ensure
that callouts such as the example above are obeyed.
THE CALLOUT INTERFACE
During matching, when PCRE2 reaches a callout point, if an external
function is set in the match context, it is called. This applies to
both normal and DFA matching. The first argument to the callout func-
tion is a pointer to a pcre2_callout block. The second argument is the
void * callout data that was supplied when the callout was set up by
calling pcre2_set_callout() (see the pcre2api documentation). The call-
out block structure contains the following fields:
uint32_t version;
uint32_t callout_number;
uint32_t capture_top;
uint32_t capture_last;
PCRE2_SIZE *offset_vector;
PCRE2_SPTR mark;
PCRE2_SPTR subject;
PCRE2_SIZE subject_length;
PCRE2_SIZE start_match;
PCRE2_SIZE current_position;
PCRE2_SIZE pattern_position;
PCRE2_SIZE next_item_length;
PCRE2_SIZE callout_string_offset;
PCRE2_SIZE callout_string_length;
PCRE2_SPTR callout_string;
The version field contains the version number of the block format. The
current version is 1; the three callout string fields were added for
this version. If you are writing an application that might use an ear-
lier release of PCRE2, you should check the version number before
accessing any of these fields. The version number will increase in
future if more fields are added, but the intention is never to remove
any of the existing fields.
Fields for numerical callouts
For a numerical callout, callout_string is NULL, and callout_number
contains the number of the callout, in the range 0-255. This is the
number that follows (?C for manual callouts; it is 255 for automati-
cally generated callouts.
Fields for string callouts
For callouts with string arguments, callout_number is always zero, and
callout_string points to the string that is contained within the com-
piled pattern. Its length is given by callout_string_length. Duplicated
ending delimiters that were present in the original pattern string have
been turned into single characters, but there is no other processing of
the callout string argument. An additional code unit containing binary
zero is present after the string, but is not included in the length.
The delimiter that was used to start the string is also stored within
the pattern, immediately before the string itself. You can access this
delimiter as callout_string[-1] if you need it.
The callout_string_offset field is the code unit offset to the start of
the callout argument string within the original pattern string. This is
provided for the benefit of applications such as script languages that
might need to report errors in the callout string within the pattern.
Fields for all callouts
The remaining fields in the callout block are the same for both kinds
of callout.
The offset_vector field is a pointer to the vector of capturing offsets
(the "ovector") that was passed to the matching function in the match
data block. When pcre2_match() is used, the contents can be inspected
in order to extract substrings that have been matched so far, in the
same way as for extracting substrings after a match has completed. For
the DFA matching function, this field is not useful.
The subject and subject_length fields contain copies of the values that
were passed to the matching function.
The start_match field normally contains the offset within the subject
at which the current match attempt started. However, if the escape
sequence \K has been encountered, this value is changed to reflect the
modified starting point. If the pattern is not anchored, the callout
function may be called several times from the same point in the pattern
for different starting points in the subject.
The current_position field contains the offset within the subject of
the current match pointer.
When the pcre2_match() is used, the capture_top field contains one more
than the number of the highest numbered captured substring so far. If
no substrings have been captured, the value of capture_top is one. This
is always the case when the DFA functions are used, because they do not
support captured substrings.
The capture_last field contains the number of the most recently cap-
tured substring. However, when a recursion exits, the value reverts to
what it was outside the recursion, as do the values of all captured
substrings. If no substrings have been captured, the value of cap-
ture_last is 0. This is always the case for the DFA matching functions.
The pattern_position field contains the offset in the pattern string to
the next item to be matched.
The next_item_length field contains the length of the next item to be
matched in the pattern string. When the callout immediately precedes an
alternation bar, a closing parenthesis, or the end of the pattern, the
length is zero. When the callout precedes an opening parenthesis, the
length is that of the entire subpattern.
The pattern_position and next_item_length fields are intended to help
in distinguishing between different automatic callouts, which all have
the same callout number. However, they are set for all callouts, and
are used by pcre2test to show the next item to be matched when display-
ing callout information.
In callouts from pcre2_match() the mark field contains a pointer to the
zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
(*THEN) item in the match, or NULL if no such items have been passed.
Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
previous (*MARK). In callouts from the DFA matching function this field
always contains NULL.
RETURN VALUES FROM CALLOUTS
The external callout function returns an integer to PCRE2. If the value
is zero, matching proceeds as normal. If the value is greater than
zero, matching fails at the current point, but the testing of other
matching possibilities goes ahead, just as if a lookahead assertion had
failed. If the value is less than zero, the match is abandoned, and the
matching function returns the negative value.
Negative values should normally be chosen from the set of
PCRE2_ERROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces a
standard "no match" failure. The error number PCRE2_ERROR_CALLOUT is
reserved for use by callout functions; it will never be used by PCRE2
itself.
CALLOUT ENUMERATION
int pcre2_callout_enumerate(const pcre2_code *code,
int (*callback)(pcre2_callout_enumerate_block *, void *),
void *user_data);
A script language that supports the use of string arguments in callouts
might like to scan all the callouts in a pattern before running the
match. This can be done by calling pcre2_callout_enumerate(). The first
argument is a pointer to a compiled pattern, the second points to a
callback function, and the third is arbitrary user data. The callback
function is called for every callout in the pattern in the order in
which they appear. Its first argument is a pointer to a callout enumer-
ation block, and its second argument is the user_data value that was
passed to pcre2_callout_enumerate(). The data block contains the fol-
lowing fields:
version Block version number
pattern_position Offset to next item in pattern
next_item_length Length of next item in pattern
callout_number Number for numbered callouts
callout_string_offset Offset to string within pattern
callout_string_length Length of callout string
callout_string Points to callout string or is NULL
The version number is currently 0. It will increase if new fields are
ever added to the block. The remaining fields are the same as their
namesakes in the pcre2_callout block that is used for callouts during
matching, as described above.
Note that the value of pattern_position is unique for each callout.
However, if a callout occurs inside a group that is quantified with a
non-zero minimum or a fixed maximum, the group is replicated inside the
compiled pattern. For example, a pattern such as /(a){2}/ is compiled
as if it were /(a)(a)/. This means that the callout will be enumerated
more than once, but with the same value for pattern_position in each
case.
The callback function should normally return zero. If it returns a non-
zero value, scanning the pattern stops, and that value is returned from
pcre2_callout_enumerate().
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 23 March 2015
Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------
PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
DIFFERENCES BETWEEN PCRE2 AND PERL
This document describes the differences in the ways that PCRE2 and Perl
handle regular expressions. The differences described here are with
respect to Perl versions 5.10 and above.
1. PCRE2 has only a subset of Perl's Unicode support. Details of what
it does have are given in the pcre2unicode page.
2. PCRE2 allows repeat quantifiers only on parenthesized assertions,
but they do not mean what you might think. For example, (?!a){3} does
not assert that the next three characters are not "a". It just asserts
that the next character is not "a" three times (in principle: PCRE2
optimizes this to run the assertion just once). Perl allows repeat
quantifiers on other assertions such as \b, but these do not seem to
have any use.
3. Capturing subpatterns that occur inside negative lookahead asser-
tions are counted, but their entries in the offsets vector are never
set. Perl sometimes (but not always) sets its numerical variables from
inside negative assertions.
4. The following Perl escape sequences are not supported: \l, \u, \L,
\U, and \N when followed by a character name or Unicode value. (\N on
its own, matching a non-newline character, is supported.) In fact these
are implemented by Perl's general string-handling and are not part of
its pattern matching engine. If any of these are encountered by PCRE2,
an error is generated by default. However, if the PCRE2_ALT_BSUX option
is set, \U and \u are interpreted as ECMAScript interprets them.
5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
is built with Unicode support. The properties that can be tested with
\p and \P are limited to the general category properties such as Lu and
Nd, script names such as Greek or Han, and the derived properties Any
and L&. PCRE2 does support the Cs (surrogate) property, which Perl does
not; the Perl documentation says "Because Perl hides the need for the
user to understand the internal representation of Unicode characters,
there is no need to implement the somewhat messy concept of surro-
gates."
6. PCRE2 does support the \Q...\E escape for quoting substrings. Char-
acters in between are treated as literals. This is slightly different
from Perl in that $ and @ are also handled as literals inside the
quotes. In Perl, they cause variable interpolation (but of course PCRE2
does not have variables). Note the following examples:
Pattern PCRE2 matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
7. Fairly obviously, PCRE2 does not support the (?{code}) and
(??{code}) constructions. However, there is support for recursive pat-
terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also,
the PCRE2 "callout" feature allows an external function to be called
during pattern matching. See the pcre2callout documentation for
details.
8. Subroutine calls (whether recursive or not) are treated as atomic
groups. Atomic recursion is like Python, but unlike Perl. Captured
values that are set outside a subroutine call can be referenced from
inside in PCRE2, but not in Perl. There is a discussion that explains
these differences in more detail in the section on recursion differ-
ences from Perl in the pcre2pattern page.
9. If any of the backtracking control verbs are used in a subpattern
that is called as a subroutine (whether or not recursively), their
effect is confined to that subpattern; it does not extend to the sur-
rounding pattern. This is not always the case in Perl. In particular,
if (*THEN) is present in a group that is called as a subroutine, its
action is limited to that group, even if the group does not contain any
| characters. Note that such subpatterns are processed as anchored at
the point where they are tested.
10. If a pattern contains more than one backtracking control verb, the
first one that is backtracked onto acts. For example, in the pattern
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
it is the same as PCRE2, but there are examples where it differs.
11. Most backtracking verbs in assertions have their normal actions.
They are not confined to the assertion.
12. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE2 it is set to "b".
13. PCRE2's handling of duplicate subpattern numbers and duplicate sub-
pattern names is not as general as Perl's. This is a consequence of the
fact the PCRE2 works internally just with numbers, using an external
table to translate between numbers and names. In particular, a pattern
such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
the same number but different names, is not supported, and causes an
error at compile time. If it were allowed, it would not be possible to
distinguish which parentheses matched, because both names map to cap-
turing subpattern number 1. To avoid this confusing situation, an error
is given at compile time.
14. Perl recognizes comments in some places that PCRE2 does not, for
example, between the ( and ? at the start of a subpattern. If the /x
modifier is set, Perl allows white space between ( and ? (though cur-
rent Perls warn that this is deprecated) but PCRE2 never does, even if
the PCRE2_EXTENDED option is set.
15. Perl, when in warning mode, gives warnings for character classes
such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter-
als. PCRE2 has no warning features, so it gives an error in these cases
because they are almost certainly user mistakes.
16. In PCRE2, the upper/lower case character properties Lu and Ll are
not affected when case-independent matching is specified. For example,
\p{Lu} always matches an upper case letter. I think Perl has changed in
this respect; in the release at the time of writing (5.16), \p{Lu} and
\p{Ll} match all letters, regardless of case, when case independence is
specified.
17. PCRE2 provides some extensions to the Perl regular expression
facilities. Perl 5.10 includes new features that are not in earlier
versions of Perl, some of which (such as named parentheses) have been
in PCRE2 for some time. This list is with respect to Perl 5.10:
(a) Although lookbehind assertions in PCRE2 must match fixed length
strings, each alternative branch of a lookbehind assertion can match a
different length of string. Perl requires them all to have the same
length.
(b) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
$ meta-character matches only at the very end of the string.
(c) A backslash followed by a letter with no special meaning is
faulted. (Perl can be made to issue a warning.)
(d) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti-
fiers is inverted, that is, by default they are not greedy, but if fol-
lowed by a question mark they are.
(e) PCRE2_ANCHORED can be used at matching time to force a pattern to
be tried only at the first matching position in the subject string.
(f) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
PCRE2_NOTEMPTY_ATSTART, and PCRE2_NO_AUTO_CAPTURE options have no Perl
equivalents.
(g) The \R escape sequence can be restricted to match only CR, LF, or
CRLF by the PCRE2_BSR_ANYCRLF option.
(h) The callout facility is PCRE2-specific.
(i) The partial matching facility is PCRE2-specific.
(j) The alternative matching function (pcre2_dfa_match() matches in a
different way and is not Perl-compatible.
(k) PCRE2 recognizes some special sequences such as (*CR) at the start
of a pattern that set overall options that cannot be changed within the
pattern.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 15 March 2015
Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------
PCRE2JIT(3) Library Functions Manual PCRE2JIT(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 JUST-IN-TIME COMPILER SUPPORT
Just-in-time compiling is a heavyweight optimization that can greatly
speed up pattern matching. However, it comes at the cost of extra pro-
cessing before the match is performed, so it is of most benefit when
the same pattern is going to be matched many times. This does not nec-
essarily mean many calls of a matching function; if the pattern is not
anchored, matching attempts may take place many times at various posi-
tions in the subject, even for a single call. Therefore, if the subject
string is very long, it may still pay to use JIT even for one-off
matches. JIT support is available for all of the 8-bit, 16-bit and
32-bit PCRE2 libraries.
JIT support applies only to the traditional Perl-compatible matching
function. It does not apply when the DFA matching function is being
used. The code for this support was written by Zoltan Herczeg.
AVAILABILITY OF JIT SUPPORT
JIT support is an optional feature of PCRE2. The "configure" option
--enable-jit (or equivalent CMake option) must be set when PCRE2 is
built if you want to use JIT. The support is limited to the following
hardware platforms:
ARM 32-bit (v5, v7, and Thumb2)
ARM 64-bit
Intel x86 32-bit and 64-bit
MIPS 32-bit and 64-bit
Power PC 32-bit and 64-bit
SPARC 32-bit
If --enable-jit is set on an unsupported platform, compilation fails.
A program can tell if JIT support is available by calling pcre2_con-
fig() with the PCRE2_CONFIG_JIT option. The result is 1 when JIT is
available, and 0 otherwise. However, a simple program does not need to
check this in order to use JIT. The API is implemented in a way that
falls back to the interpretive code if JIT is not available. For pro-
grams that need the best possible performance, there is also a "fast
path" API that is JIT-specific.
SIMPLE USE OF JIT
To make use of the JIT support in the simplest way, all you have to do
is to call pcre2_jit_compile() after successfully compiling a pattern
with pcre2_compile(). This function has two arguments: the first is the
compiled pattern pointer that was returned by pcre2_compile(), and the
second is zero or more of the following option bits: PCRE2_JIT_COM-
PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.
If JIT support is not available, a call to pcre2_jit_compile() does
nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled
pattern is passed to the JIT compiler, which turns it into machine code
that executes much faster than the normal interpretive code, but yields
exactly the same results. The returned value from pcre2_jit_compile()
is zero on success, or a negative error code.
PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for com-
plete matches. If you want to run partial matches using the PCRE2_PAR-
TIAL_HARD or PCRE2_PARTIAL_SOFT options of pcre2_match(), you should
set one or both of the other options as well as, or instead of
PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
for each of the three modes (normal, soft partial, hard partial). When
pcre2_match() is called, the appropriate code is run if it is avail-
able. Otherwise, the pattern is matched using interpretive code.
You can call pcre2_jit_compile() multiple times for the same compiled
pattern. It does nothing if it has previously compiled code for any of
the option bits. For example, you can call it once with PCRE2_JIT_COM-
PLETE and (perhaps later, when you find you need partial matching)
again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time it
will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
ing. If pcre2_jit_compile() is called with no option bits set, it imme-
diately returns zero. This is an alternative way of testing whether JIT
is available.
At present, it is not possible to free JIT compiled code except when
the entire compiled pattern is freed by calling pcre2_code_free().
In some circumstances you may need to call additional functions. These
are described in the section entitled "Controlling the JIT stack"
below.
There are some pcre2_match() options that are not supported by JIT, and
there are also some pattern items that JIT cannot handle. Details are
given below. In both cases, matching automatically falls back to the
interpretive code. If you want to know whether JIT was actually used
for a particular match, you should arrange for a JIT callback function
to be set up as described in the section entitled "Controlling the JIT
stack" below, even if you do not need to supply a non-default JIT
stack. Such a callback function is called whenever JIT code is about to
be obeyed. If the match-time options are not right for JIT execution,
the callback function is not obeyed.
If the JIT compiler finds an unsupported item, no JIT data is gener-
ated. You can find out if JIT matching is available after compiling a
pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE
option. A non-zero result means that JIT compilation was successful. A
result of 0 means that JIT support is not available, or the pattern was
not processed by pcre2_jit_compile(), or the JIT compiler was not able
to handle the pattern.
UNSUPPORTED OPTIONS AND PATTERN ITEMS
The pcre2_match() options that are supported for JIT matching are
PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The
PCRE2_ANCHORED option is not supported at match time.
The only unsupported pattern items are \C (match a single data unit)
when running in a UTF mode, and a callout immediately before an asser-
tion condition in a conditional group.
RETURN VALUES FROM JIT MATCHING
When a pattern is matched using JIT matching, the return values are the
same as those given by the interpretive pcre2_match() code, with the
addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means
that the memory used for the JIT stack was insufficient. See "Control-
ling the JIT stack" below for a discussion of JIT stack usage.
The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if
searching a very large pattern tree goes on for too long, as it is in
the same circumstance when JIT is not used, but the details of exactly
what is counted are not the same. The PCRE2_ERROR_RECURSIONLIMIT error
code is never returned when JIT matching is used.
CONTROLLING THE JIT STACK
When the compiled JIT code runs, it needs a block of memory to use as a
stack. By default, it uses 32K on the machine stack. However, some
large or complicated patterns need more than this. The error
PCRE2_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
Three functions are provided for managing blocks of memory for use as
JIT stacks. There is further discussion about the use of JIT stacks in
the section entitled "JIT stack FAQ" below.
The pcre2_jit_stack_create() function creates a JIT stack. Its argu-
ments are a starting size, a maximum size, and a general context (for
memory allocation functions, or NULL for standard memory allocation).
It returns a pointer to an opaque structure of type pcre2_jit_stack, or
NULL if there is an error. The pcre2_jit_stack_free() function is used
to free a stack that is no longer needed. (For the technically minded:
the address space is allocated by mmap or VirtualAlloc.)
JIT uses far less memory for recursion than the interpretive code, and
a maximum stack size of 512K to 1M should be more than enough for any
pattern.
The pcre2_jit_stack_assign() function specifies which stack JIT code
should use. Its arguments are as follows:
pcre2_match_context *mcontext
pcre2_jit_callback callback
void *data
The first argument is a pointer to a match context. When this is subse-
quently passed to a matching function, its information determines which
JIT stack is used. There are three cases for the values of the other
two options:
(1) If callback is NULL and data is NULL, an internal 32K block
on the machine stack is used. This is the default when a match
context is created.
(2) If callback is NULL and data is not NULL, data must be
a pointer to a valid JIT stack, the result of calling
pcre2_jit_stack_create().
(3) If callback is not NULL, it must point to a function that is
called with data as an argument at the start of matching, in
order to set up a JIT stack. If the return from the callback
function is NULL, the internal 32K stack is used; otherwise the
return value must be a valid JIT stack, the result of calling
pcre2_jit_stack_create().
A callback function is obeyed whenever JIT code is about to be run; it
is not obeyed when pcre2_match() is called with options that are incom-
patible for JIT matching. A callback function can therefore be used to
determine whether a match operation was executed by JIT or by the
interpreter.
You may safely use the same JIT stack for more than one pattern (either
by assigning directly or by callback), as long as the patterns are all
matched sequentially in the same thread. In a multithread application,
if you do not specify a JIT stack, or if you assign or pass back NULL
from a callback, that is thread-safe, because each thread has its own
machine stack. However, if you assign or pass back a non-NULL JIT
stack, this must be a different stack for each thread so that the
application is thread-safe.
Strictly speaking, even more is allowed. You can assign the same non-
NULL stack to a match context that is used by any number of patterns,
as long as they are not used for matching by multiple threads at the
same time. For example, you could use the same stack in all compiled
patterns, with a global mutex in the callback to wait until the stack
is available for use. However, this is an inefficient solution, and not
recommended.
This is a suggestion for how a multithreaded program that needs to set
up non-default JIT stacks might operate:
During thread initalization
thread_local_var = pcre2_jit_stack_create(...)
During thread exit
pcre2_jit_stack_free(thread_local_var)
Use a one-line callback function
return thread_local_var
All the functions described in this section do nothing if JIT is not
available.
JIT STACK FAQ
(1) Why do we need JIT stacks?
PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
where the local data of the current node is pushed before checking its
child nodes. Allocating real machine stack on some platforms is diffi-
cult. For example, the stack chain needs to be updated every time if we
extend the stack on PowerPC. Although it is possible, its updating
time overhead decreases performance. So we do the recursion in memory.
(2) Why don't we simply allocate blocks of memory with malloc()?
Modern operating systems have a nice feature: they can reserve an
address space instead of allocating memory. We can safely allocate mem-
ory pages inside this address space, so the stack could grow without
moving memory data (this is important because of pointers). Thus we can
allocate 1M address space, and use only a single memory page (usually
4K) if that is enough. However, we can still grow up to 1M anytime if
needed.
(3) Who "owns" a JIT stack?
The owner of the stack is the user program, not the JIT studied pattern
or anything else. The user program must ensure that if a stack is being
used by pcre2_match(), (that is, it is assigned to a match context that
is passed to the pattern currently running), that stack must not be
used by any other threads (to avoid overwriting the same memory area).
The best practice for multithreaded programs is to allocate a stack for
each thread, and return this stack through the JIT callback function.
(4) When should a JIT stack be freed?
You can free a JIT stack at any time, as long as it will not be used by
pcre2_match() again. When you assign the stack to a match context, only
a pointer is set. There is no reference counting or any other magic.
You can free compiled patterns, contexts, and stacks in any order, any-
time. Just do not call pcre2_match() with a match context pointing to
an already freed stack, as that will cause SEGFAULT. (Also, do not free
a stack currently used by pcre2_match() in another thread). You can
also replace the stack in a context at any time when it is not in use.
You should free the previous stack before assigning a replacement.
(5) Should I allocate/free a stack every time before/after calling
pcre2_match()?
No, because this is too costly in terms of resources. However, you
could implement some clever idea which release the stack if it is not
used in let's say two minutes. The JIT callback can help to achieve
this without keeping a list of patterns.
(6) OK, the stack is for long term memory allocation. But what happens
if a pattern causes stack overflow with a stack of 1M? Is that 1M kept
until the stack is freed?
Especially on embedded sytems, it might be a good idea to release mem-
ory sometimes without freeing the stack. There is no API for this at
the moment. Probably a function call which returns with the currently
allocated memory for any stack and another which allows releasing mem-
ory (shrinking the stack) would be a good idea if someone needs this.
(7) This is too much of a headache. Isn't there any better solution for
JIT stack handling?
No, thanks to Windows. If POSIX threads were used everywhere, we could
throw out this complicated API.
FREEING JIT SPECULATIVE MEMORY
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
The JIT executable allocator does not free all memory when it is possi-
ble. It expects new allocations, and keeps some free memory around to
improve allocation speed. However, in low memory conditions, it might
be better to free all possible memory. You can cause this to happen by
calling pcre2_jit_free_unused_memory(). Its argument is a general con-
text, for custom memory management, or NULL for standard memory manage-
ment.
EXAMPLE CODE
This is a single-threaded example that specifies a JIT stack without
using a callback. A real program should include error checking after
all the function calls.
int rc;
pcre2_code *re;
pcre2_match_data *match_data;
pcre2_match_context *mcontext;
pcre2_jit_stack *jit_stack;
re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
&errornumber, &erroffset, NULL);
rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
mcontext = pcre2_match_context_create(NULL);
jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
match_data = pcre2_match_data_create(re, 10);
rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
/* Process result */
pcre2_code_free(re);
pcre2_match_data_free(match_data);
pcre2_match_context_free(mcontext);
pcre2_jit_stack_free(jit_stack);
JIT FAST PATH API
Because the API described above falls back to interpreted matching when
JIT is not available, it is convenient for programs that are written
for general use in many environments. However, calling JIT via
pcre2_match() does have a performance impact. Programs that are written
for use where JIT is known to be available, and which need the best
possible performance, can instead use a "fast path" API to call JIT
matching directly instead of calling pcre2_match() (obviously only for
patterns that have been successfully processed by pcre2_jit_compile()).
The fast path function is called pcre2_jit_match(), and it takes
exactly the same arguments as pcre2_match(). The return values are also
the same, plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (partial or
complete) is requested that was not compiled. Unsupported option bits
(for example, PCRE2_ANCHORED) are ignored.
When you call pcre2_match(), as well as testing for invalid options, a
number of other sanity checks are performed on the arguments. For exam-
ple, if the subject pointer is NULL, an immediate error is given. Also,
unless PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested for
validity. In the interests of speed, these checks do not happen on the
JIT fast path, and if invalid data is passed, the result is undefined.
Bypassing the sanity checks and the pcre2_match() wrapping can give
speedups of more than 10%.
SEE ALSO
pcre2api(3)
AUTHOR
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge, England.
REVISION
Last updated: 27 November 2014
Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------
PCRE2LIMITS(3) Library Functions Manual PCRE2LIMITS(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
SIZE AND OTHER LIMITATIONS
There are some size limitations in PCRE2 but it is hoped that they will
never in practice be relevant.
The maximum size of a compiled pattern is approximately 64K code units
for the 8-bit and 16-bit libraries if PCRE2 is compiled with the
default internal linkage size, which is 2 bytes for these libraries. If
you want to process regular expressions that are truly enormous, you
can compile PCRE2 with an internal linkage size of 3 or 4 (when build-
ing the 16-bit library, 3 is rounded up to 4). See the README file in
the source distribution and the pcre2build documentation for details.
In these cases the limit is substantially larger. However, the speed
of execution is slower. In the 32-bit library, the internal linkage
size is always 4.
The maximum length (in code units) of a subject string is one less than
the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an
unsigned integer type, usually defined as size_t. Its maximum value
(that is ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-
terminated strings and unset offsets.
Note that when using the traditional matching function, PCRE2 uses
recursion to handle subpatterns and indefinite repetition. This means
that the available stack space may limit the size of a subject string
that can be processed by certain patterns. For a discussion of stack
issues, see the pcre2stack documentation.
All values in repeating quantifiers must be less than 65536.
There is no limit to the number of parenthesized subpatterns, but there
can be no more than 65535 capturing subpatterns. There is, however, a
limit to the depth of nesting of parenthesized subpatterns of all
kinds. This is imposed in order to limit the amount of system stack
used at compile time. The limit can be specified when PCRE2 is built;
the default is 250.
There is a limit to the number of forward references to subsequent sub-
patterns of around 200,000. Repeated forward references with fixed
upper limits, for example, (?2){0,100} when subpattern number 2 is to
the right, are included in the count. There is no limit to the number
of backward references.
The maximum length of name for a named subpattern is 32 code units, and
the maximum number of named subpatterns is 10000.
The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or
(*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit and
32-bit libraries.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 25 November 2014
Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------
PCRE2MATCHING(3) Library Functions Manual PCRE2MATCHING(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 MATCHING ALGORITHMS
This document describes the two different algorithms that are available
in PCRE2 for matching a compiled regular expression against a given
subject string. The "standard" algorithm is the one provided by the
pcre2_match() function. This works in the same as as Perl's matching
function, and provide a Perl-compatible matching operation. The just-
in-time (JIT) optimization that is described in the pcre2jit documenta-
tion is compatible with this function.
An alternative algorithm is provided by the pcre2_dfa_match() function;
it operates in a different way, and is not Perl-compatible. This alter-
native has advantages and disadvantages compared with the standard
algorithm, and these are described below.
When there is only one possible way in which a given subject string can
match a pattern, the two algorithms give the same answer. A difference
arises, however, when there are multiple possibilities. For example, if
the pattern
^<.*>
is matched against the string
<something> <something else> <something further>
there are three possible answers. The standard algorithm finds only one
of them, whereas the alternative algorithm finds all three.
REGULAR EXPRESSIONS AS TREES
The set of strings that are matched by a regular expression can be rep-
resented as a tree structure. An unlimited repetition in the pattern
makes the tree of infinite size, but it is still a tree. Matching the
pattern to a given subject string (from a given starting point) can be
thought of as a search of the tree. There are two ways to search a
tree: depth-first and breadth-first, and these correspond to the two
matching algorithms provided by PCRE2.
THE STANDARD MATCHING ALGORITHM
In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
sions", the standard algorithm is an "NFA algorithm". It conducts a
depth-first search of the pattern tree. That is, it proceeds along a
single path through the tree, checking that the subject matches what is
required. When there is a mismatch, the algorithm tries any alterna-
tives at the current point, and if they all fail, it backs up to the
previous branch point in the tree, and tries the next alternative
branch at that level. This often involves backing up (moving to the
left) in the subject string as well. The order in which repetition
branches are tried is controlled by the greedy or ungreedy nature of
the quantifier.
If a leaf node is reached, a matching string has been found, and at
that point the algorithm stops. Thus, if there is more than one possi-
ble match, this algorithm returns the first one that it finds. Whether
this is the shortest, the longest, or some intermediate length depends
on the way the greedy and ungreedy repetition quantifiers are specified
in the pattern.
Because it ends up with a single path through the tree, it is rela-
tively straightforward for this algorithm to keep track of the sub-
strings that are matched by portions of the pattern in parentheses.
This provides support for capturing parentheses and back references.
THE ALTERNATIVE MATCHING ALGORITHM
This algorithm conducts a breadth-first search of the tree. Starting
from the first matching point in the subject, it scans the subject
string from left to right, once, character by character, and as it does
this, it remembers all the paths through the tree that represent valid
matches. In Friedl's terminology, this is a kind of "DFA algorithm",
though it is not implemented as a traditional finite state machine (it
keeps multiple states active simultaneously).
Although the general principle of this matching algorithm is that it
scans the subject string only once, without backtracking, there is one
exception: when a lookaround assertion is encountered, the characters
following or preceding the current point have to be independently
inspected.
The scan continues until either the end of the subject is reached, or
there are no more unterminated paths. At this point, terminated paths
represent the different matching possibilities (if there are none, the
match has failed). Thus, if there is more than one possible match,
this algorithm finds all of them, and in particular, it finds the long-
est. The matches are returned in decreasing order of length. There is
an option to stop the algorithm after the first match (which is neces-
sarily the shortest) is found.
Note that all the matches that are found start at the same point in the
subject. If the pattern
cat(er(pillar)?)?
is matched against the string "the caterpillar catchment", the result
is the three strings "caterpillar", "cater", and "cat" that start at
the fifth character of the subject. The algorithm does not automati-
cally move on to find matches that start at later positions.
PCRE2's "auto-possessification" optimization usually applies to charac-
ter repeats at the end of a pattern (as well as internally). For exam-
ple, the pattern "a\d+" is compiled as if it were "a\d++" because there
is no point even considering the possibility of backtracking into the
repeated digits. For DFA matching, this means that only one possible
match is found. If you really do want multiple matches in such cases,
either use an ungreedy repeat ("a\d+?") or set the PCRE2_NO_AUTO_POS-
SESS option when compiling.
There are a number of features of PCRE2 regular expressions that are
not supported by the alternative matching algorithm. They are as fol-
lows:
1. Because the algorithm finds all possible matches, the greedy or
ungreedy nature of repetition quantifiers is not relevant (though it
may affect auto-possessification, as just described). During matching,
greedy and ungreedy quantifiers are treated in exactly the same way.
However, possessive quantifiers can make a difference when what follows
could also match what is quantified, for example in a pattern like
this:
^a++\w!
This pattern matches "aaab!" but not "aaa!", which would be matched by
a non-possessive quantifier. Similarly, if an atomic group is present,
it is matched as if it were a standalone pattern at the current point,
and the longest match is then "locked in" for the rest of the overall
pattern.
2. When dealing with multiple paths through the tree simultaneously, it
is not straightforward to keep track of captured substrings for the
different matching possibilities, and PCRE2's implementation of this
algorithm does not attempt to do this. This means that no captured sub-
strings are available.
3. Because no substrings are captured, back references within the pat-
tern are not supported, and cause errors if encountered.
4. For the same reason, conditional expressions that use a backrefer-
ence as the condition or test for a specific group recursion are not
supported.
5. Because many paths through the tree may be active, the \K escape
sequence, which resets the start of the match when encountered (but may
be on some paths and not on others), is not supported. It causes an
error if encountered.
6. Callouts are supported, but the value of the capture_top field is
always 1, and the value of the capture_last field is always 0.
7. The \C escape sequence, which (in the standard algorithm) always
matches a single code unit, even in a UTF mode, is not supported in
these modes, because the alternative algorithm moves through the sub-
ject string one character (not code unit) at a time, for all active
paths through the tree.
8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
are not supported. (*FAIL) is supported, and behaves like a failing
negative assertion.
ADVANTAGES OF THE ALTERNATIVE ALGORITHM
Using the alternative matching algorithm provides the following advan-
tages:
1. All possible matches (at a single point in the subject) are automat-
ically found, and in particular, the longest match is found. To find
more than one match using the standard algorithm, you have to do kludgy
things with callouts.
2. Because the alternative algorithm scans the subject string just
once, and never needs to backtrack (except for lookbehinds), it is pos-
sible to pass very long subject strings to the matching function in
several pieces, checking for partial matching each time. Although it is
also possible to do multi-segment matching using the standard algo-
rithm, by retaining partially matched substrings, it is more compli-
cated. The pcre2partial documentation gives details of partial matching
and discusses multi-segment matching.
DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
The alternative algorithm suffers from a number of disadvantages:
1. It is substantially slower than the standard algorithm. This is
partly because it has to search for all possible matches, but is also
because it is less susceptible to optimization.
2. Capturing parentheses and back references are not supported.
3. Although atomic groups are supported, their use does not provide the
performance advantage that it does for the standard algorithm.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 29 September 2014
Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------
PCRE2PARTIAL(3) Library Functions Manual PCRE2PARTIAL(3)
NAME
PCRE2 - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE2
In normal use of PCRE2, if the subject string that is passed to a
matching function matches as far as it goes, but is too short to match
the entire pattern, PCRE2_ERROR_NOMATCH is returned. There are circum-
stances where it might be helpful to distinguish this case from other
cases in which there is no match.
Consider, for example, an application where a human is required to type
in data for a field with specific formatting requirements. An example
might be a date in the form ddmmmyy, defined by this pattern:
^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
If the application sees the user's keystrokes one by one, and can check
that what has been typed so far is potentially valid, it is able to
raise an error as soon as a mistake is made, by beeping and not
reflecting the character that has been typed, for example. This immedi-
ate feedback is likely to be a better user interface than a check that
is delayed until the entire string has been entered. Partial matching
can also be useful when the subject string is very long and is not all
available at once.
PCRE2 supports partial matching by means of the PCRE2_PARTIAL_SOFT and
PCRE2_PARTIAL_HARD options, which can be set when calling a matching
function. The difference between the two options is whether or not a
partial match is preferred to an alternative complete match, though the
details differ between the two types of matching function. If both
options are set, PCRE2_PARTIAL_HARD takes precedence.
If you want to use partial matching with just-in-time optimized code,
you must call pcre2_jit_compile() with one or both of these options:
PCRE2_JIT_PARTIAL_SOFT
PCRE2_JIT_PARTIAL_HARD
PCRE2_JIT_COMPLETE should also be set if you are going to run non-par-
tial matches on the same pattern. If the appropriate JIT mode has not
been compiled, interpretive matching code is used.
Setting a partial matching option disables two of PCRE2's standard
optimizations. PCRE2 remembers the last literal code unit in a pattern,
and abandons matching immediately if it is not present in the subject
string. This optimization cannot be used for a subject string that
might match only partially. PCRE2 also knows the minimum length of a
matching string, and does not bother to run the matching function on
shorter strings. This optimization is also disabled for partial match-
ing.
PARTIAL MATCHING USING pcre2_match()
A partial match occurs during a call to pcre2_match() when the end of
the subject string is reached successfully, but matching cannot con-
tinue because more characters are needed. However, at least one charac-
ter in the subject must have been inspected. This character need not
form part of the final matched string; lookbehind assertions and the \K
escape sequence provide ways of inspecting characters before the start
of a matched string. The requirement for inspecting at least one char-
acter exists because an empty string can always be matched; without
such a restriction there would always be a partial match of an empty
string at the end of the subject.
When a partial match is returned, the first two elements in the ovector
point to the portion of the subject that was matched, but the values in
the rest of the ovector are undefined. The appearance of \K in the pat-
tern has no effect for a partial match. Consider this pattern:
/abc\K123/
If it is matched against "456abc123xyz" the result is a complete match,
and the ovector defines the matched string as "123", because \K resets
the "start of match" point. However, if a partial match is requested
and the subject string is "456abc12", a partial match is found for the
string "abc12", because all these characters are needed for a subse-
quent re-match with additional characters.
What happens when a partial match is identified depends on which of the
two partial matching options are set.
PCRE2_PARTIAL_SOFT WITH pcre2_match()
If PCRE2_PARTIAL_SOFT is set when pcre2_match() identifies a partial
match, the partial match is remembered, but matching continues as nor-
mal, and other alternatives in the pattern are tried. If no complete
match can be found, PCRE2_ERROR_PARTIAL is returned instead of
PCRE2_ERROR_NOMATCH.
This option is "soft" because it prefers a complete match over a par-
tial match. All the various matching items in a pattern behave as if
the subject string is potentially complete. For example, \z, \Z, and $
match at the end of the subject, as normal, and for \b and \B the end
of the subject is treated as a non-alphanumeric.
If there is more than one partial match, the first one that was found
provides the data that is returned. Consider this pattern:
/123\w+X|dogY/
If this is matched against the subject string "abc123dog", both alter-
natives fail to match, but the end of the subject is reached during
matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are set to 3
and 9, identifying "123dog" as the first partial match that was found.
(In this example, there are two partial matches, because "dog" on its
own partially matches the second alternative.)
PCRE2_PARTIAL_HARD WITH pcre2_match()
If PCRE2_PARTIAL_HARD is set for pcre2_match(), PCRE2_ERROR_PARTIAL is
returned as soon as a partial match is found, without continuing to
search for possible complete matches. This option is "hard" because it
prefers an earlier partial match over a later complete match. For this
reason, the assumption is made that the end of the supplied subject
string may not be the true end of the available data, and so, if \z,
\Z, \b, \B, or $ are encountered at the end of the subject, the result
is PCRE2_ERROR_PARTIAL, provided that at least one character in the
subject has been inspected.
Comparing hard and soft partial matching
The difference between the two partial matching options can be illus-
trated by a pattern such as:
/dog(sbody)?/
This matches either "dog" or "dogsbody", greedily (that is, it prefers
the longer string if possible). If it is matched against the string
"dog" with PCRE2_PARTIAL_SOFT, it yields a complete match for "dog".
However, if PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR-
TIAL. On the other hand, if the pattern is made ungreedy the result is
different:
/dog(sbody)??/
In this case the result is always a complete match because that is
found first, and matching never continues after finding a complete
match. It might be easier to follow this explanation by thinking of the
two patterns like this:
/dog(sbody)?/ is the same as /dogsbody|dog/
/dog(sbody)??/ is the same as /dog|dogsbody/
The second pattern will never match "dogsbody", because it will always
find the shorter match first.
PARTIAL MATCHING USING pcre2_dfa_match()
The DFA functions move along the subject string character by character,
without backtracking, searching for all possible matches simultane-
ously. If the end of the subject is reached before the end of the pat-
tern, there is the possibility of a partial match, again provided that
at least one character has been inspected.
When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
there have been no complete matches. Otherwise, the complete matches
are returned. However, if PCRE2_PARTIAL_HARD is set, a partial match
takes precedence over any complete matches. The portion of the string
that was matched when the longest partial match was found is set as the
first matching string.
Because the DFA functions always search for all possible matches, and
there is no difference between greedy and ungreedy repetition, their
behaviour is different from the standard functions when PCRE2_PAR-
TIAL_HARD is set. Consider the string "dog" matched against the
ungreedy pattern shown above:
/dog(sbody)??/
Whereas the standard function stops as soon as it finds the complete
match for "dog", the DFA function also finds the partial match for
"dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.
PARTIAL MATCHING AND WORD BOUNDARIES
If a pattern ends with one of sequences \b or \B, which test for word
boundaries, partial matching with PCRE2_PARTIAL_SOFT can give counter-
intuitive results. Consider this pattern:
/\bcat\b/
This matches "cat", provided there is a word boundary at either end. If
the subject string is "the cat", the comparison of the final "t" with a
following character cannot take place, so a partial match is found.
However, normal matching carries on, and \b matches at the end of the
subject when the last character is a letter, so a complete match is
found. The result, therefore, is not PCRE2_ERROR_PARTIAL. Using
PCRE2_PARTIAL_HARD in this case does yield PCRE2_ERROR_PARTIAL, because
then the partial match takes precedence.
EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST
If the partial_soft (or ps) modifier is present on a pcre2test data
line, the PCRE2_PARTIAL_SOFT option is used for the match. Here is a
run of pcre2test that uses the date example quoted above:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25jun04\=ps
0: 25jun04
1: jun
data> 25dec3\=ps
Partial match: 23dec3
data> 3ju\=ps
Partial match: 3ju
data> 3juj\=ps
No match
data> j\=ps
No match
The first data string is matched completely, so pcre2test shows the
matched substrings. The remaining four strings do not match the com-
plete pattern, but the first two are partial matches. Similar output is
obtained if DFA matching is used.
If the partial_hard (or ph) modifier is present on a pcre2test data
line, the PCRE2_PARTIAL_HARD option is set for the match.
MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()
When a partial match has been found using a DFA matching function, it
is possible to continue the match by providing additional subject data
and calling the function again with the same compiled regular expres-
sion, this time setting the PCRE2_DFA_RESTART option. You must pass the
same working space as before, because this is where details of the pre-
vious partial match are stored. Here is an example using pcre2test:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\=dfa,ps
Partial match: 23ja
data> n05\=dfa,dfa_restart
0: n05
The first call has "23ja" as the subject, and requests partial match-
ing; the second call has "n05" as the subject for the continued
(restarted) match. Notice that when the match is complete, only the
last part is shown; PCRE2 does not retain the previously partially-
matched string. It is up to the calling program to do that if it needs
to.
That means that, for an unanchored pattern, if a continued match fails,
it is not possible to try again at a new starting point. All this
facility is capable of doing is continuing with the previous match
attempt. In the previous example, if the second set of data is "ug23"
the result is no match, even though there would be a match for "aug23"
if the entire string were given at once. Depending on the application,
this may or may not be what you want. The only way to allow for start-
ing again at the next character is to retain the matched part of the
subject and try a new complete match.
You can set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with
PCRE2_DFA_RESTART to continue partial matching over multiple segments.
This facility can be used to pass very long subject strings to the DFA
matching functions.
MULTI-SEGMENT MATCHING WITH pcre2_match()
Unlike the DFA function, it is not possible to restart the previous
match with a new segment of data when using pcre2_match(). Instead, new
data must be added to the previous subject string, and the entire match
re-run, starting from the point where the partial match occurred. Ear-
lier data can be discarded.
It is best to use PCRE2_PARTIAL_HARD in this situation, because it does
not treat the end of a segment as the end of the subject when matching
\z, \Z, \b, \B, and $. Consider an unanchored pattern that matches
dates:
re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
data> The date is 23ja\=ph
Partial match: 23ja
At this stage, an application could discard the text preceding "23ja",
add on text from the next segment, and call the matching function
again. Unlike the DFA matching function, the entire matching string
must always be available, and the complete matching process occurs for
each call, so more memory and more processing time is needed.
ISSUES WITH MULTI-SEGMENT MATCHING
Certain types of pattern may give problems with multi-segment matching,
whichever matching function is used.
1. If the pattern contains a test for the beginning of a line, you need
to pass the PCRE2_NOTBOL option when the subject string for any call
does start at the beginning of a line. There is also a PCRE2_NOTEOL
option, but in practice when doing multi-segment matching you should be
using PCRE2_PARTIAL_HARD, which includes the effect of PCRE2_NOTEOL.
2. If a pattern contains a lookbehind assertion, characters that pre-
cede the start of the partial match may have been inspected during the
matching process. When using pcre2_match(), sufficient characters must
be retained for the next match attempt. You can ensure that enough
characters are retained by doing the following:
Before doing any matching, find the length of the longest lookbehind in
the pattern by calling pcre2_pattern_info() with the
PCRE2_INFO_MAXLOOKBEHIND option. Note that the resulting count is in
characters, not code units. After a partial match, moving back from the
ovector[0] offset in the subject by the number of characters given for
the maximum lookbehind gets you to the earliest character that must be
retained. In a non-UTF or a 32-bit situation, moving back is just a
subtraction, but in UTF-8 or UTF-16 you have to count characters while
moving back through the code units.
Characters before the point you have now reached can be discarded, and
after the next segment has been added to what is retained, you should
run the next match with the startoffset argument set so that the match
begins at the same point as before.
For example, if the pattern "(?<=123)abc" is partially matched against
the string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maxi-
mum lookbehind count is 3, so all characters before offset 2 can be
discarded. The value of startoffset for the next match should be 3.
When pcre2test displays a partial match, it indicates the lookbehind
characters with '<' characters:
re> "(?<=123)abc"
data> xx123ab\=ph
Partial match: 123ab
<<<
3. Because a partial match must always contain at least one character,
what might be considered a partial match of an empty string actually
gives a "no match" result. For example:
re> /c(?<=abc)x/
data> ab\=ps
No match
If the next segment begins "cx", a match should be found, but this will
only happen if characters from the previous segment are retained. For
this reason, a "no match" result should be interpreted as "partial
match of an empty string" when the pattern contains lookbehinds.
4. Matching a subject string that is split into multiple segments may
not always produce exactly the same result as matching over one single
long string, especially when PCRE2_PARTIAL_SOFT is used. The section
"Partial Matching and Word Boundaries" above describes an issue that
arises if the pattern ends with \b or \B. Another kind of difference
may occur when there are multiple matching possibilities, because (for
PCRE2_PARTIAL_SOFT) a partial match result is given only when there are
no completed matches. This means that as soon as the shortest match has
been found, continuation to a new subject segment is no longer possi-
ble. Consider this pcre2test example:
re> /dog(sbody)?/
data> dogsb\=ps
0: dog
data> do\=ps,dfa
Partial match: do
data> gsb\=ps,dfa,dfa_restart
0: g
data> dogsbody\=dfa
0: dogsbody
1: dog
The first data line passes the string "dogsb" to a standard matching
function, setting the PCRE2_PARTIAL_SOFT option. Although the string is
a partial match for "dogsbody", the result is not PCRE2_ERROR_PARTIAL,
because the shorter string "dog" is a complete match. Similarly, when
the subject is presented to a DFA matching function in several parts
("do" and "gsb" being the first two) the match stops when "dog" has
been found, and it is not possible to continue. On the other hand, if
"dogsbody" is presented as a single string, a DFA matching function
finds both matches.
Because of these problems, it is best to use PCRE2_PARTIAL_HARD when
matching multi-segment data. The example above then behaves differ-
ently:
re> /dog(sbody)?/
data> dogsb\=ph
Partial match: dogsb
data> do\=ps,dfa
Partial match: do
data> gsb\=ph,dfa,dfa_restart
Partial match: gsb
5. Patterns that contain alternatives at the top level which do not all
start with the same pattern item may not work as expected when
PCRE2_DFA_RESTART is used. For example, consider this pattern:
1234|3789
If the first part of the subject is "ABC123", a partial match of the
first alternative is found at offset 3. There is no partial match for
the second alternative, because such a match does not start at the same
point in the subject string. Attempting to continue with the string
"7890" does not yield a match because only those alternatives that
match at one point in the subject are remembered. The problem arises
because the start of the second alternative matches within the first
alternative. There is no problem with anchored patterns or patterns
such as:
1234|ABCD
where no string can be a partial match for both alternatives. This is
not a problem if a standard matching function is used, because the
entire match has to be rerun each time:
re> /1234|3789/
data> ABC123\=ph
Partial match: 123
data> 1237890
0: 3789
Of course, instead of using PCRE2_DFA_RESTART, the same technique of
re-running the entire match can also be used with the DFA matching
function. Another possibility is to work with two buffers. If a partial
match at offset n in the first buffer is followed by "no match" when
PCRE2_DFA_RESTART is used on the second buffer, you can then try a new
match starting at offset n+1 in the first buffer.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 22 December 2014
Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------
PCRE2UNICODE(3) Library Functions Manual PCRE2UNICODE(3)
NAME
PCRE - Perl-compatible regular expressions (revised API)
UNICODE AND UTF SUPPORT
When PCRE2 is built with Unicode support (which is the default), it has
knowledge of Unicode character properties and can process text strings
in UTF-8, UTF-16, or UTF-32 format (depending on the code unit width).
However, by default, PCRE2 assumes that one code unit is one character.
To process a pattern as a UTF string, where a character may require
more than one code unit, you must call pcre2_compile() with the
PCRE2_UTF option flag, or the pattern must start with the sequence
(*UTF). When either of these is the case, both the pattern and any sub-
ject strings that are matched against it are treated as UTF strings
instead of strings of individual one-code-unit characters.
If you do not need Unicode support you can build PCRE2 without it, in
which case the library will be smaller.
UNICODE PROPERTY SUPPORT
When PCRE2 is built with Unicode support, the escape sequences \p{..},
\P{..}, and \X can be used. The Unicode properties that can be tested
are limited to the general category properties such as Lu for an upper
case letter or Nd for a decimal number, the Unicode script names such
as Arabic or Han, and the derived properties Any and L&. Full lists are
given in the pcre2pattern and pcre2syntax documentation. Only the short
names for properties are supported. For example, \p{L} matches a let-
ter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in
Perl, many properties may optionally be prefixed by "Is", for compati-
bility with Perl 5.6. PCRE does not support this.
WIDE CHARACTERS AND UTF MODES
Codepoints less than 256 can be specified in patterns by either braced
or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
Larger values have to use braced sequences. Unbraced octal code points
up to \777 are also recognized; larger ones can be coded using \o{...}.
In UTF modes, repeat quantifiers apply to complete UTF characters, not
to individual code units.
In UTF modes, the dot metacharacter matches one UTF character instead
of a single code unit.
The escape sequence \C can be used to match a single code unit, in a
UTF mode, but its use can lead to some strange effects because it
breaks up multi-unit characters (see the description of \C in the
pcre2pattern documentation). The use of \C is not supported in the
alternative matching function pcre2_dfa_match(), nor is it supported in
UTF mode by the JIT optimization. If JIT optimization is requested for
a UTF pattern that contains \C, it will not succeed, and so the match-
ing will be carried out by the normal interpretive function.
The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
characters of any code value, but, by default, the characters that
PCRE2 recognizes as digits, spaces, or word characters remain the same
set as in non-UTF mode, all with code points less than 256. This
remains true even when PCRE2 is built to include Unicode support,
because to do otherwise would slow down matching in many common cases.
Note that this also applies to \b and \B, because they are defined in
terms of \w and \W. If you want to test for a wider sense of, say,
"digit", you can use explicit Unicode property tests such as \p{Nd}.
Alternatively, if you set the PCRE2_UCP option, the way that the char-
acter escapes work is changed so that Unicode properties are used to
determine which characters match. There are more details in the section
on generic character types in the pcre2pattern documentation.
Similarly, characters that match the POSIX named character classes are
all low-valued characters, unless the PCRE2_UCP option is set.
However, the special horizontal and vertical white space matching
escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
acters, whether or not PCRE2_UCP is set.
Case-insensitive matching in UTF mode makes use of Unicode properties.
A few Unicode characters such as Greek sigma have more than two code-
points that are case-equivalent, and these are treated as such.
VALIDITY OF UTF STRINGS
When the PCRE2_UTF option is set, the strings passed as patterns and
subjects are (by default) checked for validity on entry to the relevant
functions. If an invalid UTF string is passed, an negative error code
is returned. The code unit offset to the offending character can be
extracted from the match data block by calling pcre2_get_startchar(),
which is used for this purpose after a UTF error.
UTF-16 and UTF-32 strings can indicate their endianness by special code
knows as a byte-order mark (BOM). The PCRE2 functions do not handle
this, expecting strings to be in host byte order.
The entire string is checked before any other processing takes place.
In addition to checking the format of the string, there is a check to
ensure that all code points lie in the range U+0 to U+10FFFF, excluding
the surrogate area. The so-called "non-character" code points are not
excluded because Unicode corrigendum #9 makes it clear that they should
not be.
Characters in the "Surrogate Area" of Unicode are reserved for use by
UTF-16, where they are used in pairs to encode code points with values
greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
are available independently in the UTF-8 and UTF-32 encodings. (In
other words, the whole surrogate thing is a fudge for UTF-16 which
unfortunately messes up UTF-8 and UTF-32.)
In some situations, you may already know that your strings are valid,
and therefore want to skip these checks in order to improve perfor-
mance, for example in the case of a long subject string that is being
scanned repeatedly. If you set the PCRE2_NO_UTF_CHECK option at com-
pile time or at match time, PCRE2 assumes that the pattern or subject
it is given (respectively) contains only valid UTF code unit sequences.
Passing PCRE2_NO_UTF_CHECK to pcre2_compile() just disables the check
for the pattern; it does not also apply to subject strings. If you want
to disable the check for a subject string you must pass this option to
pcre2_match() or pcre2_dfa_match().
If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the
result is undefined and your program may crash or loop indefinitely.
Errors in UTF-8 strings
The following negative error codes are given for invalid UTF-8 strings:
PCRE2_ERROR_UTF8_ERR1
PCRE2_ERROR_UTF8_ERR2
PCRE2_ERROR_UTF8_ERR3
PCRE2_ERROR_UTF8_ERR4
PCRE2_ERROR_UTF8_ERR5
The string ends with a truncated UTF-8 character; the code specifies
how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
characters to be no longer than 4 bytes, the encoding scheme (origi-
nally defined by RFC 2279) allows for up to 6 bytes, and this is
checked first; hence the possibility of 4 or 5 missing bytes.
PCRE2_ERROR_UTF8_ERR6
PCRE2_ERROR_UTF8_ERR7
PCRE2_ERROR_UTF8_ERR8
PCRE2_ERROR_UTF8_ERR9
PCRE2_ERROR_UTF8_ERR10
The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
the character do not have the binary value 0b10 (that is, either the
most significant bit is 0, or the next bit is 1).
PCRE2_ERROR_UTF8_ERR11
PCRE2_ERROR_UTF8_ERR12
A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
long; these code points are excluded by RFC 3629.
PCRE2_ERROR_UTF8_ERR13
A 4-byte character has a value greater than 0x10fff; these code points
are excluded by RFC 3629.
PCRE2_ERROR_UTF8_ERR14
A 3-byte character has a value in the range 0xd800 to 0xdfff; this
range of code points are reserved by RFC 3629 for use with UTF-16, and
so are excluded from UTF-8.
PCRE2_ERROR_UTF8_ERR15
PCRE2_ERROR_UTF8_ERR16
PCRE2_ERROR_UTF8_ERR17
PCRE2_ERROR_UTF8_ERR18
PCRE2_ERROR_UTF8_ERR19
A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
for a value that can be represented by fewer bytes, which is invalid.
For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
rect coding uses just one byte.
PCRE2_ERROR_UTF8_ERR20
The two most significant bits of the first byte of a character have the
binary value 0b10 (that is, the most significant bit is 1 and the sec-
ond is 0). Such a byte can only validly occur as the second or subse-
quent byte of a multi-byte character.
PCRE2_ERROR_UTF8_ERR21
The first byte of a character has the value 0xfe or 0xff. These values
can never occur in a valid UTF-8 string.
Errors in UTF-16 strings
The following negative error codes are given for invalid UTF-16
strings:
PCRE_UTF16_ERR1 Missing low surrogate at end of string
PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate
PCRE_UTF16_ERR3 Isolated low surrogate
Errors in UTF-32 strings
The following negative error codes are given for invalid UTF-32
strings:
PCRE_UTF32_ERR1 Surrogate character (range from 0xd800 to 0xdfff)
PCRE_UTF32_ERR2 Code point is greater than 0x10ffff
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 23 November 2014
Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------
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