regex

A regular expression, or regex, is a compact pattern used to search, match, and manipulate strings. In theory, regexes describe regular languages and relate to finite automata; in practice, software engines interpret patterns to scan text.

Origin: The concept derives from formal language theory (Kleene, 1950s). Regex-like syntax appeared in early text tools, and modern languages and utilities extend it with varying features and dialects.

Syntax overview: Literals match exact characters; metacharacters define patterns. Common features include character classes [abc], quantifiers *, +, ?, and {m,n}, anchors ^ and $, grouping (), and alternation |. Escapes with backslash allow literal metacharacters. Lookahead and backreferences exist in many engines but are not universal.

Usage and limitations: Regexes are used for validation, extraction, substitution, and splitting. They allow concise text processing but can be slow if poorly designed; some patterns cause catastrophic backtracking. Most regex theories describe regular languages, but many engines add extensions that make matching non-regular and potentially more powerful.

Examples: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} matches a simple email-like address. \b\d{2}/\d{2}/\d{4}\b matches a date. Exact syntax varies between tools such as PCRE, Python, JavaScript, Java, and .NET.