Escaping Ordinary Characters in Regex in PHP

The modifier X causes the escaping of an ordinary character to result in an error. Without this operator, ordinary characters can be escaped, they will still represent themselves (exception: digits, they will become backreferences). Let's look at an example:

php<?php
  preg_replace('#x\@x#X', '!', 'x@x'); // error, @ is not a special character
?>