I'm still not clear on what you want.

Maybe you want:

$user_string = "[SYN, ACK]";
if ($x =~ /\Q$user_string\E/) {
   print("match\n");
} else {
   print("no match\n");
}
[download]

Maybe you want:

$user_string = "[SYN, ACK]";
if ($x =~ /(\[.*?\])/) {
   if ($1 eq $user_string) {
      print("match\n");
   } else {
      print("no match\n");
   }
} else {
   print("not even close\n");
}
[download]

By the way, use &#91; for [ and &#93; for ] in your posts. Better yet, don't escape anything and place code and code bits within <c>...</c> tags.