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

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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.

In reply to Re^3: Elegant way to split into sequences of identical chars? by jonix
in thread Elegant way to split into sequences of identical chars? by pKai

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