Utility to locate errors in regular expressions

Question

Finding out why a regular expression does not match a given string can very tedious. I would like to write a utility that identifies the sub-expression causing the non-match. My idea is to use a parser to create a tree representing the complete regular expression. Then I could simplify the expression by dropping sub-expressions one by one from right to left and from bottom to top until the remaining regex matches.
The last sub-expression dropped should be (part of) the problem.

As a first step, I am looking for a parser for Python regular expressions, or a Python regex grammar to create a parser from.

But may be my idea is flawed? Or a similar (or better) tools already exists? Any advice will be highly appreciated!

Answer 1

Try

http://laurent.riesterer.free.fr/regexp/

it shows the subexpressions which cause the match by coloring the parts. Not exacty what you want, but very intuitive and powerful. Beware this is Tcl and there might be subtle differences in RE syntax, but largely
it's the same.