Add docs for regexp functions

functions-and-operators/string-functions.md

@@ -6,7 +6,7 @@ aliases: ['/docs/dev/functions-and-operators/string-functions/','/docs/dev/refer
 
 # String Functions
 
-TiDB supports most of the [string functions](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html) available in MySQL 5.7 and some of the [functions](https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlqr/SQL-Functions.html#GUID-93EC62F8-415D-4A7E-B050-5D5B2C127009) available in Oracle 21.
+TiDB supports most of the [string functions](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html) available in MySQL 5.7, some of the [string functions](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html) available in MySQL 8.0, and some of the [functions](https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlqr/SQL-Functions.html#GUID-93EC62F8-415D-4A7E-B050-5D5B2C127009) available in Oracle 21.
 
 ## Supported functions
 
@@ -47,6 +47,10 @@ TiDB supports most of the [string functions](https://dev.mysql.com/doc/refman/5.
 | [`POSITION()`](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_position)                                               | Synonym for `LOCATE()`                                                                                                                    |
 | [`QUOTE()`](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_quote)                                                     | Escape the argument for use in an SQL statement                                                                                           |
 | [`REGEXP`](https://dev.mysql.com/doc/refman/5.7/en/regexp.html#operator_regexp)                                                               | Pattern matching using regular expressions                                                                                                |
+| [`REGEXP_INSTR()`](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#function_regexp-instr) | Return the starting index of the substring that matches the regular expression (Partly compatible with MySQL. For more details, see [Regular expression compatibility with MySQL](#regular-expression-compatibility-with-mysql)) |
+| [`REGEXP_LIKE()`](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#function_regexp-like) | Whether the string matches the regular expression (Partly compatible with MySQL. For more details, see [Regular expression compatibility with MySQL](#regular-expression-compatibility-with-mysql)) |
+| [`REGEXP_REPLACE()`](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#function_regexp-replace) | Replace substrings that match the regular expression (Partly compatible with MySQL. For more details, see [Regular expression compatibility with MySQL](#regular-expression-compatibility-with-mysql)) |
+| [`REGEXP_SUBSTR()`](https://dev.mysql.com/doc/refman/8.0/en/regexp.html#function_regexp-substr) | Return the substring that matches the regular expression (Partly compatible with MySQL. For more details, see [Regular expression compatibility with MySQL](#regular-expression-compatibility-with-mysql)) |
 | [`REPEAT()`](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_repeat)                                                   | Repeat a string the specified number of times                                                                                             |
 | [`REPLACE()`](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_replace)                                                 | Replace occurrences of a specified string                                                                                                 |
 | [`REVERSE()`](https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_reverse)                                                 | Reverse the characters in a string                                                                                                        |
@@ -73,3 +77,22 @@ TiDB supports most of the [string functions](https://dev.mysql.com/doc/refman/5.
 * `SOUNDEX()`
 * `SOUNDS LIKE`
 * `WEIGHT_STRING()`
+
+## Regular expression compatibility with MySQL
+
+The following sections describe the regular expression compatibility with MySQL.
+
+### Syntax compatibility
+
+MySQL implements regular expression using International Components for Unicode (ICU), and TiDB uses RE2. To learn the syntax differences between the two libraries, you can refer to the [ICU documentation](https://unicode-org.github.io/icu/userguide/) and [RE2 Syntax](https://github.com/google/re2/wiki/Syntax).
+
+### `match_type` compatibility
+
+The value options of `match_type` between TiDB and MySQL are:
+
+- Value options in TiDB are `"c"`, `"i"`, `"m"`, and `"s"`, and value options in MySQL are `"c"`, `"i"`, `"m"`, `"n"`, and `"u"`.
+- The `"s"` in TiDB corresponds to `"n"` in MySQL. When `"s"` is set in TiDB, the `.` character also matches line terminators (`\n`).
+
+    For example, the `SELECT REGEXP_LIKE(a, b, "n") FROM t1` in MySQL is the same as the `SELECT REGEXP_LIKE(a, b, "s") FROM t1` in TiDB.
+
+- TiDB does not support `"u"`, which means Unix-only line endings in MySQL.