- var_dump: documentation ( source)
- preg_match: documentation ( source)
<?php
$selector = 'p:before';
preg_match('~::?(?:first-(?:letter|line)|before|after|selection|value|choices|repeat-(?:item|index)|outside|alternate|(?:line-)?marker|slot\([_a-z0-9+.\\\\-]*\))~i', $selector, $matches);
var_dump($matches, $selector);