On Tue, 2011-10-25 at 18:46 -0700, Adam Katz wrote:
> On 10/24/2011 02:45 PM, Karsten Bräckelmann wrote:
> [...] though I seem to recall the SA debug output includes the
> matched text (which implies $&), though if this were important, I'm sure
> we'd have already concluded it worthwhile to do stupid things like
> surrounding entire regexps with (?=this).
Good point, the debug output indeed includes the match -- coincidentally
a fact I very recently stressed in another thread. As I said earlier, I
didn't find any use of these special match capturing variables grepping
the code. This made me curious, and digging for it quickly shows the
following in Check.pm:
# note: keep this in 'single quotes' to avoid the $ & performance hit,
# unless specifically requested by the caller. Also split the
# two chars, just to be paranoid and ensure that a buggy perl interp
# doesn't impose that hit anyway (just in case)
And the code evaluated later to the special $& match variable, only run
in debug mode:
$match = '($' . '&' . '|| "negative match")';
Given the rather explicit comment and this code, I am getting convinced
my interpretation of the perlre docs was correct. The global performance
penalty for capturing grouping ONLY applies, if these special vars are
used. It's a non-issue for normal operation [1], regardless of capturing
or non-capturing grouping.
I'd still be happy for someone with more in-depth knowledge about the
subject to confirm -- or disprove.
[1] For local rules, anyway. Regarding the bulk of the stock rules, I
still support using (?:non) capturing grouping as best-practice.
> > Not trying to be confrontational, just honestly asking and wondering
> > about the real impact. After all, the perlre docs specifically
> > mention to strongly prefer non-capturing grouping basically once
> > only -- in the warning paragraph about the special vars.
--
char *t="\10pse\0r\0dtu\0.@ghno\x4e\xc8\x79\xf4\xab\x51\x8a\x10\xf4\xf4\xc4";
main(){ char h,m=h=*t++,*x=t+2*h,c,i,l=*x,s=0; for (i=0;i>=1)||!t[s+h]){ putchar(t[s]);h=m;s=0; }}}
