2009/11/4 Shlomi Fish <shlomif@[...].il> :
>  Hi all!
> 
>  Today I ran into a problem with some parsing code of mine, and I was able to
>  reduce it into the following testcase:
> 
>  ----------------------
>  #!/usr/bin/perl
> 
>  use strict;
>  use warnings;
> 
>  my $text = "<title>";
>  pos($text) = 0;
>  print (($text =~ m{\G(?=<)}cg) ? "True" : "False");
>  print "\n";
>  print 'pos($text) = ', pos($text), "\n";
>  print (($text =~ m{\G(?=<)}cg) ? "True" : "False");
>  print "\n";
>  ----------------------
> 
>  This prints "True" and then "False". The latter surprised me. After chatting
>  with it on irc://irc.perl.org/p5p , nothingmuch gave the following
>  explanation:
> 
>  ---------------------
>  #!/usr/bin/perl
> 
>  use strict;
>  use warnings;
> 
>  use Devel::Peek;
>  no warnings 'uninitialized';
> 
>  local $\ = "\n";
> 
>  my $text = "<title>";
> 
>  print "pristine ", pos($text);
>  Dump($text);
> 
>  pos($text) = 0;
>  print "reset pos ", pos($text);
>  Dump($text);
> 
>  print (($text =~ m{\G}cg) ? "True" : "False");
>  print "after successful match pos is ", pos($text);
>  Dump($text);
> 
>  print (($text =~ m{\G}g) ? "True" : "False");
>  print "after failed match ", pos($text);
>  Dump($text);
>  ---------------------
> 
>  However, to me it seems that implementation details put aside, the code above
>  should just work and return two "True"'s.
> 
>  I've tried it with the Mandriva Cooker perl, and perl-5.8.8 and perl-5.10.x-
>  latest and they all exhibit the same bug.
> 
>  Could anyone comment on it? Is this behaviour (arguably mis-behaviour)
>  documented anywhere or can anyone give me a good reason for why it should not
>  work?

If it didnt work then s///g or m//g might result in infinite loops,
and split //, $string wouldnt work either.

It is arguable the *exact* rules for how it works are not so sane, but
at this point it is too late to change them.

cheers,
yves


-- 
perl -Mre=debug -e "/just|another|perl|hacker/"