Regular expressions (regex, regexes) are at the heart of dialplan, and used in many other parts of FreeSWITCH configuration.
They are an uber-clever way to analyze, slice, dice, and massage text strings. We in FreeSWITCH we use the best and brightest, the greatest regular expressions of them all: Perl Compatible Regular Expressions (PCRE). Isn't that beautiful? It is.
A regular expression checks if a string matches a pattern (the pattern being the regex). A regular expression can also substitute parts of the strings with something else. Also, regular expressions can "select" a part of a string, and reuse it in returning the result.
In dialplan regexes are used to define the "expression" criterion of the "condition" tag. ...
