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regular expression

regular expression

A regular expression is a way of expressing a text pattern for the purpose of matching a string. Regular expressions are often used either to extract information from a string or to verify that a string is of the correct format. When referred to, regular expressions are often abbreviated to simply "regex".

The following are examples of regular expressions:

Regex	Meaning
.	A dot matches any single character.
c.t	Matches "cat", "cbt" ... "c1t" ... "c&t" etc (with anything before the "c" and anything after the "t").
[0-9]	Matches any single character in the range 0 to 9.
[09]	Matches the single character 0 or 9 (but not 1 to 8). This can be useful if you don't know the case of something, because [aA] will match either a or A.
[0-9a-zA-Z]	Matches any single character in the range 0 to 9 or a to z or A to Z.
?	Matches any single character.
*	Matches the preceding element zero or more times.
a[0-9]*z	Will match against "az", "a1z" ... "a999z" etc.
+	Matches the preceding element one or more times.
a[0-9]+z	Will match against "a1z" ... "a999z" etc (but not "az").
{number}	Matches the preceding element the specified number of times.
a[0-1]{2}z	Will match against "a00z", "a10z", "a01z" and "a11z".
{min,max}	Matches the preceding element a minimum of "min" times at at most "max" times.
a[0-1]{1,2}z	Will match against "a0z", "a1z", "a00z", "a10z", "a01z" and "a11z".
[^...]	Inverts a match - matches anything except.
a[^0-1]z	Will match against any three letter string starting with "a", ending with "z" that is not "a0z" or "a1z".
^	Matches at the start of the line.
$	Matches at the end of the line.
\	Escape character. Allows the following character to be treated as a literal rather than having a special meaning, thus \+ matches against + rather than + having its normal meaning.
\A	Matches at the start of the string. When dealing with a single line expression \A is equivalent to ^.
\Z	Matches at the end of the string. When dealing with single line expressions \Z is equivalent to $.
\xNN	Will match against the single character with the hex code 'NN'. So \0x09 will match against a tab character and \0x20 will match against a space.
\t	Match against the TAB character (same as \x09).
\n	Match against a new-line (same as \x0a).
\r	Match against a carriage return (same as \x0d).
\f	Match against a form-feed (same as \x0c).
\a	Match against a bell character (same as \x07).
\e	Match against an escape character (same as \x16)
\r\n	Match against a carriage return line feed combination (in that order).
\w	Match against any alphanumeric character (i.e. 0 to 9, a to z, A to Z and the underscore).
\W	Match against any non-alphanumeric character.
\d	Match against any numeric character.
\D	Match against any non-numeric character.
\s	Any space (same as [ \t\n\r\f]).
\S	Match against any non-space.

Relevant links:

http://etext.lib.virginia.edu/helpsheets/regex.html - Steve Ramsay's Guide to Regular Expressions.
www.regexlib.com - Regular expression "library". If you are looking for a regular expression that its likely someone else has already put together then this is a good place to look.
www.regular-expressions.info/anchors.html
www.regular-expressions.info/tutorialcnt.html - Regular expression tutorial.
http://regexpstudio.com/TRegExpr/TRegExpr.html - Freeware Delphi regular expression library, useful for Delphi developers.

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