perl regex matching failed - regex

Question

I want to match two different string and output should come in $1 and $2,
According to me in this example, if $a is 'xy abc', then $1 should be 'xy abc' and $2 should 'abc', but 'abc' part is coming in $3.
Can you please help me to writing a regex in that $1 should have whole string and $2 should
have second part.
I am using perl 5.8.5.
my #data=('abc xy','xy abc');
foreach my $a ( #data) {
print "\nPattern= $a\n";
if($a=~/(abc (xy)|xy (abc))/) {
print "\nMatch: \$1>$1< \$2>$2< \$3>$3<\n";
}
}
Output:
perl test_reg.pl
Pattern= abc xy
Match: $1>abc xy< $2>xy< $3><
Pattern= xy abc
Match: $1>xy abc< $2>< $3>abc<

Answer 1

Can be done with:
(?|(abc (xy))|(xy (abc)))
Why even bother with capturing the whole thing? You can use $& for that.
my #data = ('abc xy', 'xy abc');
for(#data) {
print "String: '$_'\n";
if(/(?|abc (xy)|xy (abc))/) {
print "Match: \$&='$&', \$1='$1'\n";
}
}

Answer 2

Because only one of captures $2 and $3 can be defined, you can write
foreach my $item ( #data) {
print "\nPattern= $item\n";
if ($item=~/(abc (xy)|xy (abc))/) {
printf "Match: whole>%s< part>%s<\n", $1, $2 || $3;
}
}
which gives the output
Pattern= abc xy
Match: whole>abc xy< part>xy<
Pattern= xy abc
Match: whole>xy abc< part>abc<

Answer 3

If you can live with allowing more capture variables than $1 and $2, then use the substrings from the branch of the alternative that matched.
for ('abc xy', 'xy abc') {
print "[$_]:\n";
if (/(abc (xy))|(xy (abc))/) {
print " - match: ", defined $1 ? "1: [$1], 2: [$2]\n"
: "1: [$3], 2: [$4]\n";
}
else {
print " - no match\n";
}
}
Output:
[abc xy]:
- match: 1: [abc xy], 2: [xy]
[xy abc]:
- match: 1: [xy abc], 2: [abc]