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.
The construct most used in FreeSWITCH demo dialplan is as follows:
<extension name="giovanni_03">
<condition field="destination_number" expression=""^(1234)$">
<action application="log" data="WARNING this is $1"/>
</condition...