with the 'S' modifier, all that occurs is mIRC will strip control codes (as $strip() does) before passing the text to the regex engine - a side effect of this is that $regml() will return stripped results and $regml().pos will refer to positions in the stripped string. it does not affect the value of $1- in the event.

since the expression '([A-Z])' only matches a single character that is never part of a control code, the only purpose that including the 'S' modifier serves is that the values of $regml().pos will be modified in a manner that is arguably unfavourable (but that's not relevant here :P). so as it stands, /S doesn't change the behaviour of the given example, but there is the implication that control codes should not be considered, which is why i suggested $strip($1-) in place of $1-.

and yes, the lack of a $len() check looks like it was an unfortunate oversight :P although even without the check it serves as an example of a simple method for calculating the percentage of capital letters in a string - it's just a shame that it isn't documented for the benefit of newcomers to coding.