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Tuesday, January 20, 2009

MySQL Regular Expressions - Part 1


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



MySQL uses Henry Spencer's implementation of regular
expressions, which is aimed at conformance with 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 Credits.



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.



Non-trivial 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.




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A regular expression may use any of the following special
characters/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 newline).

 
mysql> SELECT "fofo" REGEXP "^f.*"; -> 1
mysql> SELECT "fo\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



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(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 be in the range from 0 to RE_DUP_MAX
(default 255), inclusive. 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.


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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, for example, 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:







Name Name Name
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.


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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


1 comment:

Robert said...

This is very good information. It is usefully for beginners like me.

Thanks

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