Question

If I had a string like <start_delim>asdf<end_delim> and I wanted to take an alphanumeric string between the delimiters and reverse it using regexes, how would I go about doing this?

My natural instinct was to use something like preg_replace("<start_delim>([a-zA-Z0-9]+)<end_delim>", strrev($1), $str), but for obvious reasons, that didn’t work.

Answer 1

You’ll have to use preg_replace_callback

$str = "<start_delim>asdf<end_delim>";
function my_callback($m) {
  return $m[1].strrev($m[2]).$m[3];
}
echo preg_replace_callback("/(<start_delim>)([a-zA-Z0-9]+)(<end_delim>)/", 'my_callback' , $str);

http://codepad.org/xP2arFZk