Regexpattern

Regexpattern is a pattern defined using a regular expression, a compact formal language used to describe sets of strings. It is used for searching, validating, extracting, and transforming text.

Core elements include literals and metacharacters. Literals match themselves; metacharacters provide repetition, choice, grouping, and anchors. Common constructs: . matches any character; ^ and $ anchor the start and end of a string; [] define character classes; () grouping; | alternation; * + ? {n,m} quantify; \ escapes special meaning and allows escaping metacharacters.

Advanced features include capturing groups, non-capturing groups (?:), backreferences, lookahead and lookbehind assertions, and Unicode support flags. Pattern greediness versus laziness is important: * is greedy by default; ? makes it lazy.

Usage: regexpatterns are used to validate formats (email, phone), extract data from text, perform replacements with backreferences, and split strings. Implementations exist in many languages and tools (Python's re, JavaScript RegExp, Java java.util.regex, PCRE). Example pattern ^[A-Za-z]+$ matches an ASCII word. A simple date pattern: ^\d{2}/\d{2}/\d{4}$.

Considerations include portability across engines, and performance concerns, especially with backtracking. Precompile patterns when used repeatedly and ensure proper escaping and Unicode awareness.

See also regular expressions and pattern matching engines.