bug#18577: Regexp I-search: [(error Stack overflow in regexp matcher)]

[Stefan Monnier]

>> Is this a defect in my regexp or in the regexp engine?
> It is fundamental to the way regexp matching works.

To clarify: it is fundamental to the way *our* regexp engine works.

As long as the regexp doesn't use backrefs, it can be matched
efficiently, without backtracking.  Of course using \(..\) (as opposed
to using \(?:..\)) can also make the problem harder since the various
different (but largely equivalent) ways to match might need to be
distinguishable via match-data.

But even tho your regexp doesn't use backrefs, and even if you replace
all \(..\) with \(?:..\), your regexp will still cause problems because
our regexp engine does not try to optimize these kinds of cases.

So you have to do it by hand.

>> If the former, how could I rewrite the regexp so that it would not hit
>> these problems?

Maybe something like:

/\*\(<insidecomment>\)*\*+/

where <insidecomment> is something like

   [^'*]\|\*+\([^/'*]\|'<afterquote>\)\|'<afterquote>

where <afterquote> is something like

   \([^'*]\|\*+[^/'*]\)*'

Tho this will still push a backtrack point for every character.
Maybe better would be something like

/\*[^'*]*\(<insidecomment>\)*\*+/

where <insidecomment> is something like

   \(\*+[^/'*]\|\**'<afterquote>\)[^'*]*

where <afterquote> is still something like

   \([^'*]\|\*+[^/'*]\)*'

so that we should only push a backtrace point when we see a * or a ' in
the comment.


        Stefan