regular expression

This term is a technical staple in computer science and software engineering. While the full phrase is standard, it is almost universally abbreviated as "regex" or "regexp" in professional conversation and documentation. The term describes a formal algebraic notation for characterizing sets of strings.

Because this is a compound noun referring to a specific tool or pattern, it is treated as a countable entity. Users typically refer to "a regular expression" when discussing a single pattern and "regular expressions" when discussing the concept or a collection of patterns.