- print_r: documentation ( source)
- preg_match_all: documentation ( source)
<?php
preg_match_all('~[\\\\a-z0-9_]~', 'a\\\\b', $matches);
print_r($matches);
preg_match_all('~[a-z0-9_\\\\]~', 'a\\\\b', $matches);
print_r($matches);
preg_match_all('~[\\d-z]~', 'a\\\\b', $matches);
print_r($matches);