Pattern matching
programming language construct for processing data conditionally on its structure
In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually must be exact: "either it will or will not be a match." The patterns generally have the form of either sequences or tree structures. Uses of pattern matching include outputting the locations (if any) of a pattern within a token sequence, to output some component of the matched pattern, and to substitute the matching pattern with some other token sequence (i.e., search and replace).
Sequence patterns (e.g., a text string) are often described using regular expressions and matched using techniques such as backtracking.
Tree patterns are used in some programming languages as a general tool to process data based on its structure, e.g. C#, F#, Haskell, Java, ML, Python, Racket, Ruby, Rust, Scala, Swift and the symbolic mathematics language Mathematica have special syntax for expressing tree patterns and a language construct for conditional execution and value retrieval based on it.
Often it is possible to give alternative patterns that are tried one by one, which yields a powerful conditional programming construct. Pattern matching sometimes includes support for guards.
History
Early programming languages with pattern matching constructs include COMIT (1957), SNOBOL (1962), which introduced pattern matching as a core, first-class language capability for string and text manipulation. This paradigm evolved into structured, tree-based data evaluation with Refal (1968), which used pattern matching to manipulate symbolic expressions.
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This entry incorporates text from “Pattern matching” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.