Regular expression

A regular expression (abbreviated regexp or regex) is a way to describe sets of characters using syntactic rules. Many programming languages use or support regular expressions. A regular expression is then used by a special program or part of a programming language. This program will either generate a parser that can be used to match expressions or it will match such expressions itself. 
A regular expression processor is used for processing a regular expression statement in terms of a grammar in a given formal language, and with that examines a text string. 
A few examples of what can be matched with regular expressions:
Regular expressions can be much more complex than these examples. Many regular expression languages also support "wildcard" characters.