Regular expressions offer a set of symbols and sintactic elements that are used to match patterns of text. Search and search/replace if one the usual job regexes are used for, but they can also serve in a wider range of situations, like testing certain conditions within text streams. Regexes are extremely powerful and are implemented in web search engines, text editors, lenguages such as awk, elisp, perl, python, tcl, php, c, c++ ... as well as in vi, Delphi, Emacs among others.