my $s = '"abcd"';
print "Try for \"$s\":\n";
if ($s =~ /^(?<!\\)"((?:(?<=\\)"|[^"])*)(?<!\\)"$/) {
  print "It matches!\n";
  print '$1: ' . $1 . "\n";
  print '$2: ' . (defined($2) ? "\$2 is defined\n" : "\$2 is NOT defined\n");
} else {
  print "It does not match!\n";
}