On Mon, May 26, 2014 at 2:53 AM, John Cowan <xxxxxx@mercury.ccil.org> wrote:

I still don't like the strong wording against backreferences; I think it should say
"beware, these are expensive, use only if needed" rather than "avoid
at all costs". The latter expression raises the question "If they
should be avoided, why are they here at all?"

You're just repeating your previous argument here, so I'll

repeat my response.

They're here for backwards-compatibility, and to avoid future

forking in case people want to use them regardless of performance.

If you really know what you're doing (and what the underlying

backtracking implementation is doing), then there are uses of

backreferences that incur only a high polynomial cost. Plus

there are games you can play like matching prime numbers

in Perl which can be amusing even if slow.

That said, the general case degenerates to exponential

backtracking, which in my book warrants as strong a warning

as I can make, waving red flags, caution: land mines, there

be dragons here, abandon all hope ye who enter here,

keep away from children, pregnant women, and drunken

machinery.

Alex