This has got to be a bug. You're not going crazy. The $DIGIT variables should NOT get altered if there is no match. This is documented:
The scope of $<digit> (and $`, $&, and $') extends to the end of the enclosing BLOCK or eval string, or to the next successful pattern match, whichever comes first.
Here's the odd thing. This program works as expected: 
use strict;

my ($f1,$f2);

($f1, $f2) = 'XaaXbbX' =~ /X(\w+)X(\w+)X/;
print "\$1 = $1; \$2 = $2\n";
print "\$f1 = $f1; \$f2 = $f2\n";

($f1, $f2) = 'XXX' =~ /X(\w+)X(\w+)X/;
print "\$1 = $1; \$2 = $2\n";
print "\$f1 = $f1; \$f2 = $f2\n";
__END__
$1 = aa; $2 = bb
$f1 = aa; $f2 = bb
$1 = aa; $2 = bb
$f1 = ; $f2 =

[download]
But this program doesn't: 
use strict;

my ($f1,$f2);

$_ = 'XaaXbbX';
($f1, $f2) = /X(\w+)X(\w+)X/; # first attempt             
print "\$1 = $1; \$2 = $2\n";
print "\$f1 = $f1; \$f2 = $f2\n";

$_ = 'XXX';
($f1, $f2) = /X(\w+)X(\w+)X/; # first attempt
print "\$1 = $1; \$2 = $2\n";
print "\$f1 = $f1; \$f2 = $f2\n";
__END__
$1 = aa; $2 = bb
$f1 = aa; $f2 = bb
$1 = XX; $2 = bb
$f1 = ; $f2 =

[download]
Hmm, it seems to have something to do with the variable. Oh, and running use re 'debug' on this code shows that the second regex NEVER GETS DONE (this is a good thing, too, since that second regex demands 5 characters at least, and there are only 3, so Perl knows not to do it).

HOLY (expletive)! I just uncovered something very bad about Perl. Please watch: 

($_ = "ABCD") =~ /(..)(..)/;
print "$1, $2\n";
$_ = "WXYZ"; 
print "$1, $2\n";
__END__
AB, CD
AB, CD

[download]
That looks fine, right? Now watch THIS: 
() = ($_ = "ABCD") =~ /(..)(..)/;
print "$1, $2\n";
$_ = "WXYZ"; 
print "$1, $2\n";
__END__
AB, CD
WX, YZ

[download]
This shows that when you (supposedly) store the returned parenthetical matches from a pattern match, Perl LINKS the digit variables to SECTIONS of the string! This is probably less than good.

This happens in 5.005_02, as well as 5.6.0. I'll submit a bug report.

japhy -- Perl and Regex Hacker

In reply to Re: Another regex variable puzzle by japhy
in thread Another regex variable puzzle by Rudif

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