By the way, here's a quick way to avoid a (slow) loop through the sentence,
Code:
alias stresslink {
  var %a
  !.echo -q $regsub($1-,/(\S*?(?:www\.|tps?://)\S*)/gi,\1,%a)
  return %a
}

For example, $stresslink($1-) will accomplish underlining of the relevant urls.

The underline (ctrl+u) code that appears before the \1 can be joined/replaced with any control code you wish to use.