- var_dump: documentation ( source)
- preg_match_all: documentation ( source)
<?php
$subject = <<<EOL
1. first
2. second
3. third
12345678901234567890
EOL;
preg_match_all('/^.{1,20}?$/im', $subject, $matches, PREG_PATTERN_ORDER);
var_dump($matches[0]);