Delimiters of Regular Expressions in PHP

Not only #, but also any other characters (except letters and digits) can act as delimiters. For example, let's take the ampersand symbols as delimiters:

php<?php
  echo preg_replace('&a+&', '!', 'text');
?>

If brackets are used, then the left delimiter is the opening bracket, and the right one is the closing bracket:

php<?php
  echo preg_replace('(a+)', '!', 'text');
?>