I have a string in which I want to replace the text [[signature]] with a given value, but because it is encoded, the text looks like %5B%5Bsignature%5D%5D.
How do I replace this using a regular expression? This snippet works, but only if the string is not encoded:
$replace = preg_replace('/\[\[signature\]\]/', 'replaced!', $html);
You have encoded the string, so just decode it then run your replace.
$html = urldecode($html);
$replace = preg_replace('/\[\[signature\]\]/', 'replaced!', $html);
You can always encode it again afterwards if you need:
$html = urlencode($html);
Non-Regex Solution
If your find/replace is really that simple then you don't even need to use regex. Just do a standard string replace:
$html = str_replace('[[signature]]', 'replaced!', $html);