What are the common pitfalls or challenges faced when using regular expressions in PHP, as highlighted in the forum thread?

One common pitfall when using regular expressions in PHP is not properly escaping special characters. To avoid this issue, it's important to use the preg_quote function to escape any special characters in the pattern.

$pattern = '/[.*+?^${}()|[\]\\]/';
$escaped_pattern = preg_quote($pattern, '/');