- var_dump: documentation ( source)
- preg_match_all: documentation ( source)
<?php
$pattern = '/((?<!\w)#\w+)/i';
$subject = <<<EOT
Blablalb #toto zdjsdf #ho_yeah
EOT;
$result = [];
preg_match_all('/((?<!\w)#\w+)/i', $subject, $result);
var_dump($result);