[iainmerrick]

I was figuring you could just parse string escapes and match brackets to identify all the block scopes very cheaply.

Regex literals seem like the main tricky bit. You're right, you definitely need a real expression parser to distinguish between "a / b" and "/regex/". That still doesn't seem very expensive though (as long as you're not actually building an AST structure, just scanning through the tokens).

Automatic semicolon insertion also looks fiddly, but I don't think it affects bracket nesting at all (unlike regexes where you could have an orphaned bracket inside the string).

Overall, digging into this, it definitely strikes me that JS's syntax is just as awkward and fiddly as its semantics. Not really surprising I guess!