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From: Jesper Harder <harder@myrealbox.com>
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Subject: Re: how would you debug complex regexp searches??
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gmayer@cs.bgu.ac.il writes:

> How would one go about debugging complicated regular expressions? Is
> there some way of single-stepping through a regexp matching, so that
> I can see all the characters that were matched?

're-builder' is quite handy for writing and debugging regexps.

It's bundled with recent versions Emacs -- read the Commentary section
of re-builder.el to learn how to use it.