Re: Re: Re^4: m//g behaves strange... (!1)

[Anonymous Monk]

tye is correct, as usual. The return value of m// when false in scalar context is !1, which is special in that both numeric and string slots are flagged on, with the string value "" and the numeric value 0. The unary ~ op will use the integer value if that flag is on, so it really sees a 0 (not a conversion of the empty string to a numeric value).

#!/usr/bin/perl -w
use strict;
use Devel::Peek;
my $x = !1;
Dump $x;

$_ = 'foo';
my $y = /x/;
Dump $y;

my $z = "";
Dump $z;
__END__
SV = PVNV(0x8143040) at 0x81293cc
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,POK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x8124658 ""\0
  CUR = 0
  LEN = 1
SV = PVNV(0x8143058) at 0x8129384
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,POK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x8126180 ""\0
  CUR = 0
  LEN = 1
SV = PV(0x811fab8) at 0x81293c0
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x8149ce0 ""\0
  CUR = 0
  LEN = 1

So we were both incorrect: the return value is neither "" nor 0 but !1.

[ysth]

You beat me out - I had an almost identical reply ready when I saw yours. Let me just add that ~ tries to use an integer value even if only a floating-point value is set, and even if it is out of integer range (which can result in some strange results.)

Also, for any of the bitwise/stringwise ops (~, &, ^, |), quote the variable to force stringwise semanics if you are unsure where it has been:

$ perl -wle'$x="abc"; print ~$x; print $x+0; print ~$x; print ~"$x"'
z?o
Argument "abc" isn't numeric in addition (+) at -e line 1.
0
18446744073709551615
z?o