Lookbehind in PHP Regex

In the previous lesson, we covered lookahead. Similarly, there is positive lookbehind - (?<= ). In the following example, the replacement will occur only if 'aaa' is preceded by 'x':

php<?php
  preg_replace('#(?<=x)aaa#', '!', 'xaaa'); // returns 'x!'
?>

And there is also negative lookbehind - (?<! ). In the following example, the replacement will occur only if 'aaa' is not preceded by 'x':

php<?php
  preg_replace('#(?<!x)aaa#', '!', 'baaa'); // returns 'b!'
?>

php<?php
  $str = '$aaa $bbb $ccc';
?>