- preg_match: documentation ( source)
<?php
preg_match('/<!--\s+(?<closer>\/)?wp:(?<namespace>[a-z][a-z0-9_-]*\/)?(?<name>[a-z][a-z0-9_-]*)\s+(?<attrs>{(?:(?:[^}]+|}+(?=})|(?!}\s+\/?-->).)*+)?}\s+)?(?<void>\/)?-->/s',
'hi',
$matches,
PREG_OFFSET_CAPTURE);