From: "Tikhon B." <lists@...>
Date: 2013-02-23T00:24:49+09:00
Subject: Re: Questionable regex performance when using lazy matching and inverted character classes

Robert Klemme wrote in post #1098439:
> So, yes, there is a cost for backtracking involved, but it only shows
> only in particular situations:
> - large inputs
> - specific expressions (absence of anchoring, using unlimited
> repetition operators vs. limited (e.g. /a{0,10}/).
>
> The phenomenon is well known with regular expression engines I
> believe.  The cases you mention seem to be rather the exception than
> the common state of affairs.  Which doesn't mean that Ruby's engine
> cannot be improved.  I just don't think it's not that dramatic an
> issue.
>
> Kind regards
>
> robert

I took another look at the anchors ruby provides, and I found that the
\G anchor actually solves the issue outlined in this particular example
as it applies in my real use case to my satisfaction.

I'm still a bit unsettled that there is such a caveat to consider when
writing a relatively benign expression. However, given what the symptoms
of the problem imply about the engine design, I suspect fixing the issue
would take quite a bit of either hackery, or a fair amount of re-design.

I'll chalk it up to yet another one of those strange landmines you
encounter in ruby every once in a while.

Thanks,

Tikhon

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