Pattern matching

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 has to 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, ML, Python, 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. Early programming languages with pattern matching constructs include COMIT (1957), SNOBOL (1962), Refal (1968) with tree-based pattern matching, Prolog (1972), SASL (1976), NPL (1977), and KRC (1981). Many text editors support pattern matching of various kinds: the QED editor supports regular expression search, and some versions of TECO support the OR operator in searches. Computer algebra systems generally support pattern matching on algebraic expressions. Regular expression#History The simplest pattern in pattern matching is an explicit value or a variable. For an example, consider a simple function definition in Haskell syntax (function parameters are not in parentheses but are separated by spaces, = is not assignment but definition): f 0 = 1 Here, 0 is a single value pattern.