Regular expressions

A regular expression (abbreviated as regexp or regex, with plural forms regexps,
regexes, or regexen) is a string that describes or matches a set of strings,
according to certain syntax rules. Regular expressions are used by many text editors
and utilities to search and manipulate bodies of text based on certain patterns.

Basic concepts

A regular expression, often called a pattern, is an expression that describes
a set of strings. They are usually used to give a concise description of a set,
without having to list all elements. For example, the set containing the three
strings Handel, Händel, and Haendel can be described by the pattern "H(ä|ae?)ndel"
(or alternatively, it is said that the pattern matches each of the three strings).
In most formalisms, if there is any regex that matches a particular set then there
is an infinite number of such expressions. Most formalisms provide the following
operations to construct regular expressions.

wikipedia reference

Tools

See Handy links: Power-Search-Tools.