A great tool to learn RegEx is
The RegEx Coach.
The Regex Coach is a graphical application for Linux and Windows which can be used to experiment with (Perl-compatible) regular expressions interactively. It has the following features:
- It shows whether a regular expression matches a particular target string.
- It can also show which parts of the target string correspond to captured register groups or to arbitrary parts of the regular expression.
- It can "walk" through the target string one match at a time.
- It can simulate Perl's split and s/// (substitution) operators.
- It tries to describe the regular expression in plain English.
- It can show a graphical representation of the regular expression's parse tree.
- It can single-step through the matching process as performed by the regex engine.
- Everything happens in "real time", i.e. as soon as you make a change somewhere in the application all other parts are instantly updated.
Best of all, it's free!!