Add some string functions doc (#965)

docs/documentation/cn/sql-reference/sql-functions/string-functions/regexp_replace.md

@@ -0,0 +1,27 @@
+# regexp_replace
+
+## Syntax
+
+`VARCHAR regexp_replace(VARCHAR str, VARCHAR pattern, VARCHAR repl)
+
+## Description
+
+对字符串 str 进行正则匹配, 将命中 pattern 的部分使用 repl 来进行替换
+
+## Examples
+
+```
+mysql> SELECT regexp_replace('a b c', " ", "-");
++-----------------------------------+
+| regexp_replace('a b c', ' ', '-') |
++-----------------------------------+
+| a-b-c                             |
++-----------------------------------+
+
+mysql> SELECT regexp_replace('a b c','(b)','<\\1>');
++----------------------------------------+
+| regexp_replace('a b c', '(b)', '<\1>') |
++----------------------------------------+
+| a <b> c                                |
++----------------------------------------+
+```