1. /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 <henry@zoo.toronto.edu>. From Jargon Dictionary

2. See regular expression. From Linux Guide @FirstLinux