Re^3: Elegant way to split into sequences of identical chars?

Please try this to see more of what is going on here:

use warnings;
use strict;
use re 'debug';

my $re = qr/(.)\1*/;
my @matches; 
my $string = "xx556xx";

@matches = $string =~ m/($re)/g;
print "@matches\n\n"; # x x x x 5 5 5 5 6 6 x x x x

@matches = $string =~ m/((??{$re}))/g;
print "@matches\n\n"; # xx 55 6 xx
[download]

It looks like a question of greedy star vs. returning the first possible match and if \1 is evaluated before returning the match to me, but I dont feel much confidence to guess any further.

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Re^4: Elegant way to split into sequences of identical chars? by xdg (Monsignor) on Nov 30, 2005 at 21:14 UTC
Great suggestion! I forgot about regex debug mode. It looks like whatever the code in `(??{ code })` evaluates to is compiled into a regex (if it isn't one already) and then that regex is applied in a separate scope from the rest of the regex -- so it has its own match variables and so on. I wouldn't have expected that given the documentation in perlre. In the debug output, the telltales appear to be "Entering embedded..." and "restoring...", as seen below. Entering embedded `(.)\1' Setting an EVAL scope, savestack=31 0 <> <xx556xx> \| 1: OPEN1 0 <> <xx556xx> \| 3: REG_ANY 1 <x> <x556xx> \| 4: CLOSE1 1 <x> <x556xx> \| 6: CURLYX[1] {0,32767} 1 <x> <x556xx> \| 10: WHILEM[1/1] 0 out of 0..32767 cc=140fafc Setting an EVAL scope, savestack=37 1 <x> <x556xx> \| 8: REF1 2 <xx> <556xx> \| 10: WHILEM[1/1] 1 out of 0..32767 cc=140fafc Setting an EVAL scope, savestack=43 2 <xx> <556xx> \| 8: REF1 failed... failed, try continuation... 2 <xx> <556xx> \| 11: NOTHING 2 <xx> <556xx> \| 12: END Setting an EVAL scope, savestack=47 restoring \1 to -1(0)..1(no) restoring \1..\1 to undef [download] -xdg Code written by xdg and posted on PerlMonks is public domain. It is provided as is* with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.	[reply] [d/l] [select]
Re^4: Elegant way to split into sequences of identical chars? by Roy Johnson (Monsignor) on Nov 30, 2005 at 18:14 UTC
Another curiosity: it is a special feature of m//g that if you don't parenthesize anything, it will return the entire match, so removing the parentheses from your first example changes nothing. However, removing the outer parentheses from your second example causes a crash on my box (ActiveState 5.8.7). Caution: Contents may have been coded under pressure.	[reply]
Re^5: Elegant way to split into sequences of identical chars? by jonix (Friar) on Nov 30, 2005 at 19:48 UTC
Just tried this with perl v5.8.7 on my gentoo linux box at home: `/$re/g` matches `x 5 6 x` `/(??{$re})/g` matches `xx 55 6 xx` Cheers, jonix	[reply] [d/l] [select]