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Re^4: Can we make $& better? (need)

by tye (Sage)
on Sep 12, 2007 at 19:12 UTC ( #638652=note: print w/replies, xml ) Need Help??

in reply to Re^3: Can we make $& better? (need)
in thread Can we make $& better?

Wow. There needs to be a cheat sheet for this because every time it comes up I find mistakes, in this case my own. So my previous discovery that capturing parens cause the entire string to be copied is mostly not true. I assume the copy is required for s/// and s///g. Testing shows that otherwise the copy only happens for m// on modern perls (the 'scalar context' part doesn't appear to matter in my current testing) and doesn't even happen then with 5.6.

It is nice that I can stop fearing the performance impact of capturing parens when doing the usual scalar-context m/.../gc of a parser. Thanks!

- tye        

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Re^5: Can we make $& better? (need)
by demerphq (Chancellor) on Sep 12, 2007 at 19:24 UTC

    Hrm, ill have to check the code in more detail, however I'm pretty sure that /g in list context copies the string *once*, wheras /g in scalar context doesn't copy it at all. The latter I know because i "fixed" it in 5.9.5 and then had to back out the fix because it caused an unacceptable slowdown.

    However as far as I know you were correct, using capturing parens will cause a copy of the full string. Ill try to follow up with more details from the code when I have more time.


      BTW, my test was to change the string used in the match and then see if $1 also changed. That isn't a perfect test because it might be that the string is copied but $1 doesn't correctly use the copy made, for example. Here is a more polished version and two sets of results:

      #!/usr/bin/perl -w use strict; print "$] $^O\n"; for( qw( s l sg lg ) ) { my $x= "-y1-y2-"; my @y; $y[0]= $x =~ /(.\d)/ if 's' eq $_; @y= $x =~ /(.\d)/ if 'l' eq $_; $y[0]= $x =~ /(.\d)/g if 'sg' eq $_; @y= $x =~ /(.\d)/g if 'lg' eq $_; $x= ":1n:2n:"; print "$_ ($1)\n"; } __END__ 5.006 MSWin32 s (y1) l (1n) sg (1n) lg (2n) 5.008008 MSWin32 s (y1) l (y1) sg (1n) lg (2n)

      Which shows I'd missed the only "will copy" case in 5.6.

      - tye        

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