[Mastering-perl] Why doesn't this work?

[EMaki]

(resending, I think I sent from wrong address)

Looks like an optimisation: the /g in list context is going to
exhaust the matches.  In scalar or void context, /g will act as
an iterator, so pos() needs to persist and be mutable.

In the first case, there's no reason to bother with setting
pos(), so perl can avoid upgrading $_ to PVMG:

[ eric.maki ] $ perl -MDevel::Peek
$_ = "Just another Perl hacker,";
print Dump $_;
my( @foo ) = $_ =~ /(Just)/g;
print Dump $_;
__END__
SV = PV(0x661da8) at 0x660e7c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x6885b0 "Just another Perl hacker,"\0
  CUR = 25
  LEN = 26
SV = PV(0x661da8) at 0x660e7c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x6885b0 "Just another Perl hacker,"\0
  CUR = 25
  LEN = 26
[ eric.maki ] $ perl -MDevel::Peek
$_ = "Just another Perl hacker,";
print Dump $_;
/(Just)/g;
print Dump $_;
__END__
SV = PV(0x661da8) at 0x660e7c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x683890 "Just another Perl hacker,"\0
  CUR = 25
  LEN = 26
SV = PVMG(0x674e98) at 0x660e7c
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x683890 "Just another Perl hacker,"\0
  CUR = 25
  LEN = 26
  MAGIC = 0x6751d8
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = PERL_MAGIC_regex_global(g)
    MG_LEN = 4

Eric

---- original message : 2006-12-15 6:12pm : Andy Armstrong ----


>  From Ch1:

>

> # Doesn't actually work - the list assignment to $word seems to kill

> pos($_)

>

> 	$_ = "Just another Perl hacker,";

> 	my $pos = pos( $_ );            # same as pos()

> 	print "I'm at position $pos\n"; # -1

>

> 	my( $word ) = /(Just)/g;

> 	$pos = pos();

> 	print "[$word] ends at position $pos\n"; # 4

>

> # This works fine

>

>      $_ = "Just another Perl hacker,";

>      my $pos = pos( $_ );            # same as pos()

>      print "I'm at position $pos\n"; # undef

>      /(Just)/g;

>      $pos = pos();

>      print "[$1] ends at position $pos\n"; # 4

>

> Tested on 5.8.8 Ubuntu and 5.8.6 Mac OS X.

>

>