How do I clear string when it gets to certain length and doesn't contain the text I want?

So I'm trying to get it so my player has to type stop in order to stop a thing frokm happening. I'm using a string to append to when the letter is typed, but how would I clear the text if they got it wrong?

So you can compare your text with your max length, if it longer AND find your text in the string.

If it does not match your 2 conditions, set the text to "".

But what event lets me compare max length? I've tried using compare variable, but that doesn't work because it sees me as asking if the string is the number not the limit

Something like.

Works like a charm thanks. Just wish I didn't have to use a text object