One of the unsung successes in standardization in computer science has been the regular expression (often shortened to regex), a language for specifying text search regular expression strings. This practical language is used in every computer language, word processor, and text processing tools like the Unix tools grep or Emacs. Formally, a regular expression is an algebraic notation for characterizing a set of strings. Regular expressions are particularly useful for searching in texts, when we have a pattern to search corpus for and a corpus of texts to search through. A regular expression search function.
will search through the corpus, returning all texts that match the pattern. The corpus
can be a single document or a collection. For example, the Unix command-line tool
grep takes a regular expression and returns every line of the input document that
matches the expression.