Regex is an abbreviation of regular expression.

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Regex is a C regular expression library written by Henry Spencer. Later versions support the POSIX standardized regular expressions.

Regex is now found in various places:

Bundled with application such as Apache.
Default regular expression implementation in BSD Unix.
ftp://ftp.zoo.toronto.edu/pub/regex.shar - a shar archive, released by Henry Spencer in 1999.
http://arglist.com/regex/ -- a repackaged version for Unix systems.
Other sites around the Internet have ports or compiled code for other systems such as Windows.
Tcl includes a rewritten regular expression library by Henry Spencer.

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The GNU project has in the past published a package named Regex: this code may now be in the GNU C Library.