Revert "FIX: word boundary regex (\b) not working in Unicode languages. (#9163)"

Lookbehind regex is not supported in Firefox or IE11

This reverts commit 572bb5988f4a46bc5f66f7e9d4a09b7241c532fb.

test/javascripts/lib/highlight-text-test.js.es6

@@ -0,0 +1,26 @@
+import highlightText, { CLASS_NAME } from "discourse/lib/highlight-text";
+import { fixture } from "helpers/qunit-helpers";
+
+QUnit.module("lib:highlight-text");
+
+QUnit.test("highlighting text", assert => {
+  fixture().html(
+    `
+    <p>This is some text to highlight</p>
+    `
+  );
+
+  highlightText(fixture(), "some text");
+
+  const terms = [];
+
+  fixture(`.${CLASS_NAME}`).each((_, elem) => {
+    terms.push(elem.textContent);
+  });
+
+  assert.equal(
+    terms.join(" "),
+    "some text",
+    "it should highlight the terms correctly"
+  );
+});