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Appendix G MySQL 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. Credits. MySQL
uses the extended version to support pattern-matching operations
performed with the `REGEXP' operator in SQL statements. Pattern
matching.
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.
`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.
`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 standard class names are:
`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
These 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
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