an introduction to formal languages and automata

For speed, Thompson implemented regular expression matching by just-in-time compilation (JIT) to IBM 7094 code on the Compatible Time-Sharing System, an important early example of JIT compilation. Among the first appearances of regular expressions in program form was when Ken Thompson built Kleene's notation into the editor QED as a means to match patterns in text files. Regular expressions entered popular use from 1968 in two uses: pattern matching in a text editor and lexical analysis in a compiler. Other early implementations of pattern matching include the SNOBOL language, which did not use regular expressions, but instead its own pattern matching constructs. These arose in theoretical computer science, in the subfields of automata theory (models of computation) and the description and classification of formal languages. Regular expressions originated in 1951, when mathematician Stephen Cole Kleene described regular languages using his mathematical notation called regular events. Stephen Cole Kleene, who introduced the concept

Most general-purpose programming languages support regex capabilities either natively or via libraries, including Python, C, C++, Java, Rust, OCaml, and JavaScript. Regular expressions are used in search engines, in search and replace dialogs of word processors and text editors, in text processing utilities such as sed and AWK, and in lexical analysis. Different syntaxes for writing regular expressions have existed since the 1980s, one being the POSIX standard and another, widely used, being the Perl syntax. They came into common use with Unix text-processing utilities. The concept of regular expressions began in the 1950s, when the American mathematician Stephen Cole Kleene formalized the concept of a regular language. Regular expression techniques are developed in theoretical computer science and formal language theory. Usually such patterns are used by string-searching algorithms for 'find' or 'find and replace' operations on strings, or for input validation.

) (?= ) At least two spaces are matched, but only if they occur directly after a period (.) and before an uppercase letter.Ī regular expression (shortened as regex or regexp sometimes referred to as rational expression ) is a sequence of characters that specifies a search pattern in text.

An introduction to formal languages and automata