14.8.2 Regular Expressions

Table 14.14 Regular Expression Functions and Operators

Name	Description
NOT REGEXP	Negation of REGEXP
REGEXP	Whether string matches regular expression
REGEXP_INSTR()	Starting index of substring matching regular expression
REGEXP_LIKE()	Whether string matches regular expression
REGEXP_REPLACE()	Replace substrings matching regular expression
REGEXP_SUBSTR()	Return substring matching regular expression
RLIKE	Whether string matches regular expression

A regular expression is a powerful way of specifying a pattern for a complex search. This section discusses the functions and operators available for regular expression matching and illustrates, with examples, some of the special characters and constructs that can be used for regular expression operations. See also Section 5.3.4.7, “Pattern Matching”.

MySQL implements regular expression support using International Components for Unicode (ICU), which provides full Unicode support and is multibyte safe.

Use of a binary string with any of the MySQL regular expression functions is rejected with ER_CHARACTER_SET_MISMATCH.

• Regular Expression Function and Operator Descriptions

• Regular Expression Syntax

• Regular Expression Resource Control

Regular Expression Function and Operator Descriptions

• expr NOT REGEXP pat, expr NOT RLIKE pat

  This is the same as NOT (expr REGEXP pat).

• expr REGEXP pat, expr RLIKE pat

  Returns 1 if the string expr matches the regular expression specified by the pattern pat, 0 otherwise. If expr or pat is NULL, the return value is NULL.

  REGEXP and RLIKE are synonyms for REGEXP_LIKE().

  For additional information about how matching occurs, see the description for REGEXP_LIKE().

  mysql> SELECT 'Michael!' REGEXP '.*';
  +------------------------+
  | 'Michael!' REGEXP '.*' |
  +------------------------+
  |                      1 |
  +------------------------+
  mysql> SELECT 'new*\n*line' REGEXP 'new\\*.\\*line';
  +---------------------------------------+
  | 'new*\n*line' REGEXP 'new\\*.\\*line' |
  +---------------------------------------+
  |                                     0 |
  +---------------------------------------+
  mysql> SELECT 'a' REGEXP '^[a-d]';
  +---------------------+
  | 'a' REGEXP '^[a-d]' |
  +---------------------+
  |                   1 |
  +---------------------+

• REGEXP_INSTR(expr, pat[, pos[, occurrence[, return_option[, match_type]]]])

  Returns the starting index of the substring of the string expr that matches the regular expression specified by the pattern pat, 0 if there is no match. If expr or pat is NULL, the return value is NULL. Character indexes begin at 1.

  REGEXP_INSTR() takes these optional arguments:

  • pos: The position in expr at which to start the search. If omitted, the default is 1.

  • occurrence: Which occurrence of a match to search for. If omitted, the default is 1.

  • return_option: Which type of position to return. If this value is 0, REGEXP_INSTR() returns the position of the matched substring's first character. If this value is 1, REGEXP_INSTR() returns the position following the matched substring. If omitted, the default is 0.

  • match_type: A string that specifies how to perform matching. The meaning is as described for REGEXP_LIKE().

  For additional information about how matching occurs, see the description for REGEXP_LIKE().

  mysql> SELECT REGEXP_INSTR('dog cat dog', 'dog');
  +------------------------------------+
  | REGEXP_INSTR('dog cat dog', 'dog') |
  +------------------------------------+
  |                                  1 |
  +------------------------------------+
  mysql> SELECT REGEXP_INSTR('dog cat dog', 'dog', 2);
  +---------------------------------------+
  | REGEXP_INSTR('dog cat dog', 'dog', 2) |
  +---------------------------------------+
  |                                     9 |
  +---------------------------------------+
  mysql> SELECT REGEXP_INSTR('aa aaa aaaa', 'a{2}');
  +-------------------------------------+
  | REGEXP_INSTR('aa aaa aaaa', 'a{2}') |
  +-------------------------------------+
  |                                   1 |
  +-------------------------------------+
  mysql> SELECT REGEXP_INSTR('aa aaa aaaa', 'a{4}');
  +-------------------------------------+
  | REGEXP_INSTR('aa aaa aaaa', 'a{4}') |
  +-------------------------------------+
  |                                   8 |
  +-------------------------------------+

• REGEXP_LIKE(expr, pat[, match_type])

  Returns 1 if the string expr matches the regular expression specified by the pattern pat, 0 otherwise. If expr or pat is NULL, the return value is NULL.

  The pattern can be an extended regular expression, the syntax for which is discussed in Regular Expression Syntax. The pattern need not be a literal string. For example, it can be specified as a string expression or table column.

  The optional match_type argument is a string that may contain any or all the following characters specifying how to perform matching:

  • c: Case-sensitive matching.

  • i: Case-insensitive matching.

  • m: Multiple-line mode. Recognize line terminators within the string. The default behavior is to match line terminators only at the start and end of the string expression.

  • n: The . character matches line terminators. The default is for . matching to stop at the end of a line.

  • u: Unix-only line endings. Only the newline character is recognized as a line ending by the ., ^, and $ match operators.

  If characters specifying contradictory options are specified within match_type, the rightmost one takes precedence.

  By default, regular expression operations use the character set and collation of the expr and pat arguments when deciding the type of a character and performing the comparison. If the arguments have different character sets or collations, coercibility rules apply as described in Section 12.8.4, “Collation Coercibility in Expressions”. Arguments may be specified with explicit collation indicators to change comparison behavior.

  mysql> SELECT REGEXP_LIKE('CamelCase', 'CAMELCASE');
  +---------------------------------------+
  | REGEXP_LIKE('CamelCase', 'CAMELCASE') |
  +---------------------------------------+
  |                                     1 |
  +---------------------------------------+
  mysql> SELECT REGEXP_LIKE('CamelCase', 'CAMELCASE' COLLATE utf8mb4_0900_as_cs);
  +------------------------------------------------------------------+
  | REGEXP_LIKE('CamelCase', 'CAMELCASE' COLLATE utf8mb4_0900_as_cs) |
  +------------------------------------------------------------------+
  |                                                                0 |
  +------------------------------------------------------------------+

  match_type may be specified with the c or i characters to override the default case sensitivity. Exception: If either argument is a binary string, the arguments are handled in case-sensitive fashion as binary strings, even if match_type contains the i character.

  Note

  MySQL uses C escape syntax in strings (for example, \n to represent the newline character). If you want your expr or pat argument to contain a literal \, you must double it. (Unless the NO_BACKSLASH_ESCAPES SQL mode is enabled, in which case no escape character is used.)

  mysql> SELECT REGEXP_LIKE('Michael!', '.*');
  +-------------------------------+
  | REGEXP_LIKE('Michael!', '.*') |
  +-------------------------------+
  |                             1 |
  +-------------------------------+
  mysql> SELECT REGEXP_LIKE('new*\n*line', 'new\\*.\\*line');
  +----------------------------------------------+
  | REGEXP_LIKE('new*\n*line', 'new\\*.\\*line') |
  +----------------------------------------------+
  |                                            0 |
  +----------------------------------------------+
  mysql> SELECT REGEXP_LIKE('a', '^[a-d]');
  +----------------------------+
  | REGEXP_LIKE('a', '^[a-d]') |
  +----------------------------+
  |                          1 |
  +----------------------------+

  mysql> SELECT REGEXP_LIKE('abc', 'ABC');
  +---------------------------+
  | REGEXP_LIKE('abc', 'ABC') |
  +---------------------------+
  |                         1 |
  +---------------------------+
  mysql> SELECT REGEXP_LIKE('abc', 'ABC', 'c');
  +--------------------------------+
  | REGEXP_LIKE('abc', 'ABC', 'c') |
  +--------------------------------+
  |                              0 |
  +--------------------------------+

• REGEXP_REPLACE(expr, pat, repl[, pos[, occurrence[, match_type]]])

  Replaces occurrences in the string expr that match the regular expression specified by the pattern pat with the replacement string repl, and returns the resulting string. If expr, pat, or repl is NULL, the return value is NULL.

  REGEXP_REPLACE() takes these optional arguments:

  • pos: The position in expr at which to start the search. If omitted, the default is 1.

  • occurrence: Which occurrence of a match to replace. If omitted, the default is 0 (which means “replace all occurrences”).

  • match_type: A string that specifies how to perform matching. The meaning is as described for REGEXP_LIKE().

  The result returned by this function uses the character set and collation of the expression searched for matches.

  For additional information about how matching occurs, see the description for REGEXP_LIKE().

  mysql> SELECT REGEXP_REPLACE('a b c', 'b', 'X');
  +-----------------------------------+
  | REGEXP_REPLACE('a b c', 'b', 'X') |
  +-----------------------------------+
  | a X c                             |
  +-----------------------------------+
  mysql> SELECT REGEXP_REPLACE('abc def ghi', '[a-z]+', 'X', 1, 3);
  +----------------------------------------------------+
  | REGEXP_REPLACE('abc def ghi', '[a-z]+', 'X', 1, 3) |
  +----------------------------------------------------+
  | abc def X                                          |
  +----------------------------------------------------+

• REGEXP_SUBSTR(expr, pat[, pos[, occurrence[, match_type]]])

  Returns the substring of the string expr that matches the regular expression specified by the pattern pat, NULL if there is no match. If expr or pat is NULL, the return value is NULL.

  REGEXP_SUBSTR() takes these optional arguments:

  • pos: The position in expr at which to start the search. If omitted, the default is 1.

  • occurrence: Which occurrence of a match to search for. If omitted, the default is 1.

  • match_type: A string that specifies how to perform matching. The meaning is as described for REGEXP_LIKE().

  The result returned by this function uses the character set and collation of the expression searched for matches.

  For additional information about how matching occurs, see the description for REGEXP_LIKE().

  mysql> SELECT REGEXP_SUBSTR('abc def ghi', '[a-z]+');
  +----------------------------------------+
  | REGEXP_SUBSTR('abc def ghi', '[a-z]+') |
  +----------------------------------------+
  | abc                                    |
  +----------------------------------------+
  mysql> SELECT REGEXP_SUBSTR('abc def ghi', '[a-z]+', 1, 3);
  +----------------------------------------------+
  | REGEXP_SUBSTR('abc def ghi', '[a-z]+', 1, 3) |
  +----------------------------------------------+
  | ghi                                          |
  +----------------------------------------------+

Regular Expression Syntax

A regular expression describes a set of strings. The simplest regular expression is one that has no special characters in it. For example, the regular expression hello matches hello and nothing else.

Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regular expression hello|world contains the | alternation operator and matches either the hello or world.

As a more complex example, the regular expression B[an]*s matches any of the strings Bananas, Baaaaas, Bs, and any other string starting with a B, ending with an s, and containing any number of a or n characters in between.

The following list covers some of the basic special characters and constructs that can be used in regular expressions. For information about the full regular expression syntax supported by the ICU library used to implement regular expression support, visit the International Components for Unicode web site.

• ^

  Match the beginning of a string.

  mysql> SELECT REGEXP_LIKE('fo\nfo', '^fo$');                   -> 0
  mysql> SELECT REGEXP_LIKE('fofo', '^fo');                      -> 1

• $

  Match the end of a string.

  mysql> SELECT REGEXP_LIKE('fo\no', '^fo\no$');                 -> 1
  mysql> SELECT REGEXP_LIKE('fo\no', '^fo$');                    -> 0

• .

  Match any character (including carriage return and newline, although to match these in the middle of a string, the m (multiple line) match-control character or the (?m) within-pattern modifier must be given).

  mysql> SELECT REGEXP_LIKE('fofo', '^f.*$');                    -> 1
  mysql> SELECT REGEXP_LIKE('fo\r\nfo', '^f.*$');                -> 0
  mysql> SELECT REGEXP_LIKE('fo\r\nfo', '^f.*$', 'm');           -> 1
  mysql> SELECT REGEXP_LIKE('fo\r\nfo', '(?m)^f.*$');           -> 1

• a*

  Match any sequence of zero or more a characters.

  mysql> SELECT REGEXP_LIKE('Ban', '^Ba*n');                     -> 1
  mysql> SELECT REGEXP_LIKE('Baaan', '^Ba*n');                   -> 1
  mysql> SELECT REGEXP_LIKE('Bn', '^Ba*n');                      -> 1

• a+

  Match any sequence of one or more a characters.

  mysql> SELECT REGEXP_LIKE('Ban', '^Ba+n');                     -> 1
  mysql> SELECT REGEXP_LIKE('Bn', '^Ba+n');                      -> 0

• a?

  Match either zero or one a character.

  mysql> SELECT REGEXP_LIKE('Bn', '^Ba?n');                      -> 1
  mysql> SELECT REGEXP_LIKE('Ban', '^Ba?n');                     -> 1
  mysql> SELECT REGEXP_LIKE('Baan', '^Ba?n');                    -> 0

• de|abc

  Alternation; match either of the sequences de or abc.

  mysql> SELECT REGEXP_LIKE('pi', 'pi|apa');                     -> 1
  mysql> SELECT REGEXP_LIKE('axe', 'pi|apa');                    -> 0
  mysql> SELECT REGEXP_LIKE('apa', 'pi|apa');                    -> 1
  mysql> SELECT REGEXP_LIKE('apa', '^(pi|apa)$');                -> 1
  mysql> SELECT REGEXP_LIKE('pi', '^(pi|apa)$');                 -> 1
  mysql> SELECT REGEXP_LIKE('pix', '^(pi|apa)$');                -> 0

• (abc)*

  Match zero or more instances of the sequence abc.

  mysql> SELECT REGEXP_LIKE('pi', '^(pi)*$');                    -> 1
  mysql> SELECT REGEXP_LIKE('pip', '^(pi)*$');                   -> 0
  mysql> SELECT REGEXP_LIKE('pipi', '^(pi)*$');                  -> 1

• {1}, {2,3}

  Repetition; {n} and {m,n} notation provide a more general way of writing regular expressions that match many occurrences of the previous atom (or “piece”) of the pattern. m and n are integers.

  • a*

    Can be written as a{0,}.

  • a+

    Can be written as a{1,}.

  • a?

    Can be written as a{0,1}.

  To be more precise, a{n} matches exactly n instances of a. a{n,} matches n or more instances of a. a{m,n} matches m through n instances of a, inclusive. If both m and n are given, m must be less than or equal to n.

  mysql> SELECT REGEXP_LIKE('abcde', 'a[bcd]{2}e');              -> 0
  mysql> SELECT REGEXP_LIKE('abcde', 'a[bcd]{3}e');              -> 1
  mysql> SELECT REGEXP_LIKE('abcde', 'a[bcd]{1,10}e');           -> 1

• [a-dX], [^a-dX]

  Matches any character that is (or is not, if ^ is used) either a, b, c, d or X. A - character between two other characters forms a range that matches all characters from the first character to the second. For example, [0-9] matches any decimal digit. To include a literal ] character, it must immediately follow the opening bracket [. To include a literal - character, it must be written first or last. Any character that does not have a defined special meaning inside a [] pair matches only itself.

  mysql> SELECT REGEXP_LIKE('aXbc', '[a-dXYZ]');                 -> 1
  mysql> SELECT REGEXP_LIKE('aXbc', '^[a-dXYZ]$');               -> 0
  mysql> SELECT REGEXP_LIKE('aXbc', '^[a-dXYZ]+$');              -> 1
  mysql> SELECT REGEXP_LIKE('aXbc', '^[^a-dXYZ]+$');             -> 0
  mysql> SELECT REGEXP_LIKE('gheis', '^[^a-dXYZ]+$');            -> 1
  mysql> SELECT REGEXP_LIKE('gheisa', '^[^a-dXYZ]+$');           -> 0

• [=character_class=]

  Within a bracket expression (written using [ and ]), [=character_class=] represents an equivalence class. It matches all characters with the same collation value, including itself. For example, if o and (+) are the members of an equivalence class, [[=o=]], [[=(+)=]], and [o(+)] are all synonymous. An equivalence class may not be used as an endpoint of a range.

• [:character_class:]

  Within a bracket expression (written using [ and ]), [:character_class:] represents a character class that matches all characters belonging to that class. The following table lists the standard class names. These names stand for the character classes defined in the ctype(3) manual page. A particular locale may provide other class names. A character class may not be used as an endpoint of a range.

  Character Class Name	Meaning
  alnum	Alphanumeric characters
  alpha	Alphabetic characters
  blank	Whitespace characters
  cntrl	Control characters
  digit	Digit characters
  graph	Graphic characters
  lower	Lowercase alphabetic characters
  print	Graphic or space characters
  punct	Punctuation characters
  space	Space, tab, newline, and carriage return
  upper	Uppercase alphabetic characters
  xdigit	Hexadecimal digit characters

  mysql> SELECT REGEXP_LIKE('justalnums', '[[:alnum:]]+');       -> 1
  mysql> SELECT REGEXP_LIKE('!!', '[[:alnum:]]+');               -> 0

To use a literal instance of a special character in a regular expression, precede it by two backslash (\) characters. The MySQL parser interprets one of the backslashes, and the regular expression library interprets the other. For example, to match the string 1+2 that contains the special + character, only the last of the following regular expressions is the correct one:

mysql> SELECT REGEXP_LIKE('1+2', '1+2');                       -> 0
mysql> SELECT REGEXP_LIKE('1+2', '1\+2');                      -> 0
mysql> SELECT REGEXP_LIKE('1+2', '1\\+2');                     -> 1

Regular Expression Resource Control

REGEXP_LIKE() and similar functions use resources that can be controlled by setting system variables:

• The match engine uses memory for its internal stack. To control the maximum available memory for the stack in bytes, set the regexp_stack_limit system variable.

• The match engine operates in steps. To control the maximum number of steps performed by the engine (and thus indirectly the execution time), set the regexp_time_limit system variable. Because this limit is expressed as number of steps, it affects execution time only indirectly. Typically, it is on the order of milliseconds.