$str = '  ab+cd   (12)34';
   $str = quotemeta($str);
   $str =~ s/[a-z]/\[a-z\]/ig;
   $str =~ s/\d/\\d/g;
   $str =~ s/\s+/\\s\+/g;
   $pat = qr/$str/i;
   print "$pat\n";

   Output:
   (?i-xsm:\\s+\\s+[a-z][a-z]\+[a-z][a-z]\\s+\\s+\\s+\(\d\d\)\d\d)