Appendix G. Regular Expressions
A regular expression is a powerful way of specifying a pattern for a
complex search.
MySQL uses Henry Spencer's implementation of regular expressions,
which is aimed at conformance with POSIX 1003.2. See
Appendix C, Credits. MySQL uses the extended version to
support pattern-matching operations performed with the
REGEXP operator in SQL statements. See
Section 3.3.4.7, “Pattern Matching”, and
Section 12.3.1, “String Comparison Functions”.
This appendix is a summary, with examples, of the special characters
and constructs that can be used in MySQL for
REGEXP operations. It does not contain all the
details that can be found in Henry Spencer's
regex(7) manual page. That manual page is
included in MySQL source distributions, in the
regex.7 file under the
regex directory.
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.
Non-trivial regular expressions use certain special constructs so
that they can match more than one string. For example, the regular
expression hello|word matches either the string
hello or the string word.
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.
A regular expression for the REGEXP operator may
use any of the following special characters and constructs:
-
^
Match the beginning of a string.
mysql> SELECT 'fo\nfo' REGEXP '^fo$'; -> 0
mysql> SELECT 'fofo' REGEXP '^fo'; -> 1
-
$
Match the end of a string.
mysql> SELECT 'fo\no' REGEXP '^fo\no$'; -> 1
mysql> SELECT 'fo\no' REGEXP '^fo$'; -> 0
-
.
Match any character (including carriage return and newline).
mysql> SELECT 'fofo' REGEXP '^f.*$'; -> 1
mysql> SELECT 'fo\r\nfo' REGEXP '^f.*$'; -> 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}
{n} or {m,n} notation
provides 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.
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.
m and n must be in the
range from 0 to RE_DUP_MAX
(default 255), inclusive. If both m and
n are given, m must be
less than or equal to n.
mysql> SELECT 'abcde' REGEXP 'a[bcd]{2}e'; -> 0
mysql> SELECT 'abcde' REGEXP 'a[bcd]{3}e'; -> 1
mysql> SELECT 'abcde' REGEXP '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 '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.]
Within a bracket expression (written using [
and ]), matches the sequence of characters of
that collating element. characters is either
a single character or a character name like
newline. You can find the full list of
character names in the regexp/cname.h file.
mysql> SELECT '~' REGEXP '[[.~.]]'; -> 1
mysql> SELECT '~' REGEXP '[[.tilde.]]'; -> 1
-
[=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,
then [[=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.
mysql> SELECT 'justalnums' REGEXP '[[:alnum:]]+'; -> 1
mysql> SELECT '!!' REGEXP '[[:alnum:]]+'; -> 0
-
[[:<:]], [[:>:]]
These markers stand for word boundaries. They match the
beginning and end of words, respectively. A word is a sequence
of word characters that is not preceded by or followed by word
characters. A word character is an alphanumeric character in the
alnum class or an underscore
(_).
mysql> SELECT 'a word a' REGEXP '[[:<:]]word[[:>:]]'; -> 1
mysql> SELECT 'a xword a' REGEXP '[[:<:]]word[[:>:]]'; -> 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 '1+2' REGEXP '1+2'; -> 0
mysql> SELECT '1+2' REGEXP '1\+2'; -> 0
mysql> SELECT '1+2' REGEXP '1\\+2'; -> 1
