The Jargon File (version 4.4.7, 29 Dec 2003):

regexp
 /reg'eksp/, n.

    [Unix] (alt.: regex or reg-ex)

    1. Common written and spoken abbreviation for regular expression, one of
    the wildcard patterns used, e.g., by Unix utilities such as grep(1), sed(1)
    , and awk(1). These use conventions similar to but more elaborate than
    those described under glob. For purposes of this lexicon, it is
    sufficient to note that regexps also allow complemented character sets
    using ^; thus, one can specify ?any non-alphabetic character? with [^
    A-Za-z].

    2. Name of a well-known PD regexp-handling package in portable C, written
    by revered Usenetter Henry Spencer.


The Free On-line Dictionary of Computing (30 December 2018):

regexp

   1. regular expression.

   2. Name of a well-known PD regexp-handling package in portable
   C, written by revered Usenetter Henry Spencer
   .

   [Jargon File]