While practicing my newly acquired wisdom, I tried this 
#! perl -w
use strict;
$|++;

my $source = 'blahperlblahjavablah';
my ($f1, $f2) = $source =~ /blah(\w+)blah(\w+)blah/; # first attempt

print "#1==$1==$2==\n";
print "#2==$f1==$f2==\n";

$source = 'blahblahblah';
($f1, $f2) = $source =~ /blah(\w+)blah(\w+)blah/; # second attempt

print "#3==$1==$2==\n";
print "#4==$f1==$f2==\n";

printf "#5==%s\n", unpack "H*", "$2";

[download]
which produced the following output
H:\devperl\perlmonks>scoping-assign.pl
#1==perl==java==
#2==perl==java==
#3==blah== ava==
#4======
#5==00617661

[download]
OK, prints #1 and #2 confirm the obvious: the first attempt produces 2 successful matches, reflected both in variables $1, $2 and in my variables $f1, $f2.
The second attempt produces no matches, which is is reflected correctly in my variables $f1, $f2 (shown in #4), while the variables $1, $2 would be unchanged - or so I expected.

But, look, $2 was changed, as shown in #3, and confirmed in #5: the first character was replaced by ascii 0!

Can anyone confirm and explain this? I believed that $1, etc, variables stay unchanged when the second match attempt is not successful. But it seems that they can contain garbage.

Obtained on Win2k, AS perl 5.6.0 build 623.

Rudif

In reply to Another regex variable puzzle by Rudif

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