REGEXP

RegExp, short for regular expression, is a sequence of characters that specifies a search pattern. It is used to test whether strings contain a pattern, extract parts of text, or perform substitutions and splits. The concept originates in formal language theory and was popularized by tools such as grep; the term was coined by Stephen Cole Kleene in 1951. Modern languages ship regex engines with varying features, including Perl-compatible, JavaScript, Python, and PCRE engines. Most engines support literals, metacharacters, character classes, quantifiers, grouping and capturing, alternation, and anchors.

Common metacharacters include . to match any character, ^ and $ to anchor the pattern to string ends, [] for character classes, () for grouping, and | for alternation. Quantifiers such as *, +, ?, and {n,m} control repetition. Escaping allows treating metacharacters as literals. Flags or modifiers such as i (ignore case), g or m (global or multiline), s (single-line), and u (unicode) alter matching behavior.

Typical uses include validating input like emails or phone numbers, extracting values from text, or performing replacements. Example patterns: ^\d{3}-\d{2}-\d{4}$ to match a hyphenated number; or [A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,} for simple emails. In many languages, regexes are used via functions such as match, search, replace, or split.

Regex engines trade expressiveness for performance and can suffer from backtracking bottlenecks on certain patterns. They cannot recognize arbitrary nested structures; for such tasks, parsing with a proper parser is often required. Still, regexes remain a versatile tool across software development.