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34.3 Regular Expressions

A regular expression, or regexp for short, is a pattern that denotes a (possibly infinite) set of strings. Searching for matches for a regexp is a very powerful operation. This section explains how to write regexps; the following section says how to search for them.

For interactive development of regular expressions, you can use the M-x re-builder command. It provides a convenient interface for creating regular expressions, by giving immediate visual feedback in a separate buffer. As you edit the regexp, all its matches in the target buffer are highlighted. Each parenthesized sub-expression of the regexp is shown in a distinct face, which makes it easier to verify even very complex regexps.

Syntax of Regexps  Rules for writing regular expressions.
Regexp Example  Illustrates regular expression syntax.
Rx Notation  An alternative, structured regexp notation.
Regexp Functions  Functions for operating on regular expressions.