Regular expression

A regular expression is a formal notation for describing regular languages using operations such as union, concatenation, and repetition. It can also be interpreted as a pattern for matching strings.

Kleene star: An operation that permits zero or more repetitions of every string in a language. It expresses the repetition operator that makes regular expressions unbounded in length.
Formal language: A set of finite strings over a specified alphabet. A regular expression denotes a set of strings, rather than a single string.
POSIX Basic Regular Expressions: A standardized regular-expression syntax with rules for matching patterns in text. It is one defined practical syntax descended from formal regular-expression notation.
Context-free grammar: A grammar whose production rules replace a single nonterminal with a sequence of symbols. Context-free grammars can describe nested structures beyond regular languages.
Kleene plus: An operation that permits one or more repetitions of every string in a language. It is a common shorthand for repetition that excludes the empty sequence.
Alphabet (formal languages): A finite set of symbols from which strings in a formal language are formed. The symbols define the raw material over which an expression describes strings.
Perl Compatible Regular Expressions: A family of pattern-matching syntax and libraries based on Perl’s regular-expression features. PCRE adds widely used conveniences beyond the core formal operators.
Backreference: A pattern-matching feature that matches text previously captured by a group. Backreferences can express constraints unavailable to formal regular expressions.
Finite automaton: A machine with finitely many states that recognizes strings by following transitions. Every regular expression describes a language that some finite automaton recognizes.
Formal grammar: A system of rules that specifies how strings in a language can be generated. Grammars offer a generative perspective on languages also described by expressions.

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