Online Regex Tester and Regex Cheat Sheet

A .NET online regex tester: http://regexstorm.net/tester

RexEgg is a site containing black belt stuff concerning regexes and also a regex cheat sheet.

Another site containing fine info about Regexes.

One more, called Regextutorial.

Shorthands for character classes and anchors

Regex What it Matches Inversion
\d [0-9] or with Unicode all digits. Not in AWK. \D
\w [A-Za-z0-9_] or with Unicode all word characters \W
\s [ \t\r\n\f] or all whitespace \S
\b AWK:\y Matches the boundary at the start or end of a word. Is \y in AWK as there \b means backspace. \B

Some specialties

All of these specialties do not work in AWK nor in GAWK. They do work in Perl, probably.

Regex Name What it Does
(?:bar) non-capturing Matches bar, but doesn’t capture it.
(?=foo) lookahead Asserts that what immediately follows the current position in the string is foo.
(?<=bar) lookbehind Asserts that what immediately precedes the current position in the string is bar.
(?!baz) negative lookahead Asserts that what immediately follows the current position in the string is not baz.
(?<!foo) negative lookbehind Asserts that what immediately precedes the current position in the string is not foo.
(?i) case-insensitive Turns on case-insensitive matching. Not in AWK. AWK has no really short way to match case-insensitive. One can use BEGIN{IGNORECASE=1} as starting block or tolower($0)~/abc/ instead of /abc/
(?m) multi-line Turns on multi-line mode. $ and ^ match at any line, not only at begin and start of text. Not in AWK.
(?x) comment Activates comment-mode. Whitespace is ignored and a # in a line starts a comment. Not in AWK.
\b AWK:\y word boundary Matches the boundary at the start or end of a word. \y in AWK as there \b means backspace.