Question

Consider the code below, why it is not working?

<?php

$str = "
<h4>
   title
</h4>
";

$result = preg_match_all ('/<h4>([\d\D])<\/h4>/mi', $str, $matches);
var_dump($matches);

Answer 1

You probably meant

$str = "
<h4>
   title
</h4>
";

$result = preg_match_all ('/<h4>(.+?)<\/h4>/si', $str, $matches);
var_dump($matches);

The regex you applied, '/<h4>([\d\D])<\/h4>/mi', means “Match an opening h4, one character that’s either a digit or not a digit, and a closing h4.” But you have plenty of characters to match, so you need to specify a quantifier (“more than one”, +). Update: you need a non-greedy quantifier, +?, if you have more than one h4 (very likely!) And the class [\d\D] can be reduced to “any character”, .. One more point: you need to use /s instead of /m to get the behaviour you want.

This will probably include the newlines in your match!