H Description of MySQL regular expression syntax

A regular expression (regex) is a powerful way of specifying a complex search.

MySQL uses regular Henry Spencer's inplementation of regular expressions. And that is aimed to conform to POSIX 1003.2. MySQL uses the extended version.

This is a simplistic reference that skips the details. To get more exact information, see Henry Spencer's regex(7) manual page that is included in the source distribution. See section C Contributors to MySQL.

A regular expression describes a set of strings. The simplest regexp is one that has no special characters in it. For example, the regexp 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 regexp hello|word matches either the string hello or the string word.

As a more complex example, the regexp 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.

A regular expression may use any of the following special characters/constructs:

^

Match the beginning of a string.

mysql> select "fonfo" REGEXP "^fo$";           -> 0
mysql> select "fofo" REGEXP "^fo";              -> 1

$

Match the end of a string.

mysql> select "fono" REGEXP "^fono$";         -> 1
mysql> select "fono" REGEXP "^fo$";            -> 0

.

Match any character (including newline).

mysql> select "fofo" REGEXP "^f.*";             -> 1
mysql> select "fonfo" REGEXP "^f.*";           -> 1

a*

Match any sequence of zero or more a characters.

mysql> select "Ban" REGEXP "^Ba*n";             -> 1
mysql> select "Baaan" REGEXP "^Ba*n";           -> 1
mysql> select "Bn" REGEXP "^Ba*n";              -> 1

a+

Match any sequence of one or more a characters.

mysql> select "Ban" REGEXP "^Ba+n";             -> 1
mysql> select "Bn" REGEXP "^Ba+n";              -> 0

a?

Match either zero or one a character.

mysql> select "Bn" REGEXP "^Ba?n";              -> 1
mysql> select "Ban" REGEXP "^Ba?n";             -> 1
mysql> select "Baan" REGEXP "^Ba?n";            -> 0

de|abc

Match either of the sequences de or abc.

mysql> select "pi" REGEXP "pi|apa";             -> 1
mysql> select "axe" REGEXP "pi|apa";            -> 0
mysql> select "apa" REGEXP "pi|apa";            -> 1
mysql> select "apa" REGEXP "^(pi|apa)$";        -> 1
mysql> select "pi" REGEXP "^(pi|apa)$";         -> 1
mysql> select "pix" REGEXP "^(pi|apa)$";        -> 0

(abc)*

Match zero or more instances of the sequence abc.

mysql> select "pi" REGEXP "^(pi)*$";            -> 1
mysql> select "pip" REGEXP "^(pi)*$";           -> 0
mysql> select "pipi" REGEXP "^(pi)*$";          -> 1

{1}

{2,3}

The is a more general way of writing regexps that match many occurrences of the previous atom.

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, an atom followed by a bound containing one integer i and no comma matches a sequence of exactly i matches of the atom. An atom followed by a bound containing one integer i and a comma matches a sequence of i or more matches of the atom. An atom followed by a bound containing two integers i and j matches a sequence of i through j (inclusive) matches of the atom. Both arguments must 0 >= value <= RE_DUP_MAX (default 255). If there are two arguments, the second must be greater than or equal to the first.

[a-dX]

[^a-dX]

Matches any character which is (or is not, if ^ is used) either a, b, c, d or X. To include a literal ] character, it must immediately follow the opening bracket [. To include a literal - character, it must be written first or last. So [0-9] matches any decimal digit. Any character that does not have a defined meaning inside a [] pair has no special meaning and matches only itself.

mysql> select "aXbc" REGEXP "[a-dXYZ]";         -> 1
mysql> select "aXbc" REGEXP "^[a-dXYZ]$";       -> 0
mysql> select "aXbc" REGEXP "^[a-dXYZ]+$";      -> 1
mysql> select "aXbc" REGEXP "^[^a-dXYZ]+$";     -> 0
mysql> select "gheis" REGEXP "^[^a-dXYZ]+$";    -> 1
mysql> select "gheisa" REGEXP "^[^a-dXYZ]+$";   -> 0

[[.characters.]]

The sequence of characters of that collating element. The sequence is a single element of the bracket expression's list. A bracket expression containing a multi-character collating element can thus match more than one character, e.g., if the collating sequence includes a ch collating element, then the regular expression [[.ch.]]*c matches the first five characters of chchcc.

[=character_class=]

An equivalence class, standing for the sequences of characters of all collating elements equivalent to that one, including itself. For example, if o and (+) are the members of an equivalence class, then [[=o=]], [[=(+)=]], and [o(+)] are all synonymous. An equivalence class may not be an endpoint of a range.

[:character_class:]

Within a bracket expression, the name of a character class enclosed in [: and :] stands for the list of all characters belonging to that class. Standard character class names are:

alnum	digit	punct
alpha	graph	space
blank	lower	upper
cntrl	print	xdigit

These stand for the character classes defined in the ctype(3) manual page. A locale may provide others. A character class may not be used as an endpoint of a range.

mysql> select "justalnums" REGEXP "[[:alnum:]]+";       -> 1
mysql> select "!!" REGEXP "[[:alnum:]]+";               -> 0

[[:<:]]

[[:>:]]

These match the null string at the beginning and end of a word respectively. A word is defined as a sequence of word characters which is neither preceded nor followed by word characters. A word character is an alnum character (as defined by ctype(3)) or an underscore (_).

mysql> select "a word a" REGEXP "[[:<:]]word[[:>:]]";      -> 1
mysql> select "a xword a" REGEXP "[[:<:]]word[[:>:]]";     -> 0

mysql> select "weeknights" REGEXP "^(wee|week)(knights|nights)$"; -> 1

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