> It's an optimization. Assuming the majority of characters will be
> ASCII, most non-identifier characters will fail this test, thus
> avoiding the more involved generic Unicode check.

I don't know what kind of characters are usually received as input.  If things like (ASCII) spaces, parentheses, commas are common, then the optimization is probably OK.

@Martin
Do you know the reason why characters with the Other_ID_Start have been included in the first place, given that they are no longer considered valid identifiers and I can hardly think any situation where someone would need it?  Could they be removed from 3.5 if that makes the code simpler?