regular expression

regular expression

A regular expression is a way of expressing a text pattern for the purpose of matching a string or part of 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 table provides a summary of the different types of regular expressions together with examples of their use:

Regex	Meaning
.	A dot matches any single character - except a carriage return or line feed.
c.t	Matches "cat", "cbt" ... "c1t" ... "c&t" etc (with anything before the "c" and anything after the "t").
[0-9]	Matches anything inside the square brackets against a single character; so in this case any single character in the range 0 to 9.
[09]	Matches anything inside the square brackets against a single character; in this case 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 the preceding element one time or zero times. This is equivalent to `{0,1}`. It can be thought of as making the previous element optional.
??	Like ?, it matches the previous element 0 or 1 times, but weighted towards matching it 0 times rather than 1 if possible.
*	Matches the preceding element zero or more times.
*?	Matches the preceding element zero or more times, but unlike * will match against the shortest possible match. For example "`b[an]a`" when applied to the word "banana" will match the entire word, but "`b[an]?a`" will match with just "ban".
a[0-9]*z	Will match against "az", "a1z" ... "a999z" etc.
+	Matches the preceding element one or more times.
+?	Matches the preceding element one or more times, but unlike + will match against the shortest possible substring.
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 `\+` would match against `+` rather than `+` having its normal meaning. This also means that to include a `\` as a slash you would need to use `\\` (to escape the slash).
\A	Matches at the start of the string. When dealing with a single line expression `\A` is equivalent to `^`. Note: `\A` is not supported on all implementations of regex.
\Z	Matches at the end of the string. When dealing with single line expressions `\Z` is equivalent to `$`. Note: `\Z` is not supported on all implementations of regex.
\xNN	Will match against the single character with the hex code 'NN'. So `\x09` will match against a tab character and `\x20` will match against a space. See www.cryer.co.uk/brian/misc/ascii_table.htm for a list of hex codes for ASCII characters.
\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`).
\b	Match against a word boundary. A word boundary will match against any of: (i.) The beginning of the string, (ii.) the end of the string or (iii.) anything which is not `[a-zA-Z0-9_]`
\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.
one\|two	Match everything to the left of the bar ('\|') or everything to the right, so in this case "one" or "two".
( … )	Brackets define a group or sub-expression. When a regular expression is used in a situation where the contents of a match need to be extracted then brackets define the group or sub-expression the matching value of which can be retrieved.
\b(one\|two)\b	Match against the whole word "one" or the word "two", specifically it matches against a word boundary, then "one" or "two", followed by another word boundary.
(?<!a)b	Look behind - matches against "b" but only if the previous character was not "a". So "`(?<!a)b`" will match against "rubble" but not against "table".
(?<=a)b	Look behind - matches against "b" but only if the previous character was "a". So "`(?<!a)b`" will match against "table" but not against "rubble"

For more information see:

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 - Article on the start and end of line anchors, and the effects of line breaks.
www.regular-expressions.info/tutorialcnt.html - Regular expression tutorial.
http://everything.explained.at/Regular_expression/ - Regular expression explained.
http://msdn.microsoft.com/en-us/library/ms972966.aspx - Regular Expressions in ASP.NET, an article from Microsoft.
http://regex.cryer.info - Online regular expression evaluation tool. Lets you try out a regular expression online.