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If I worked at Facebook and was evil, I'd put some text in that span that caused catastrophic backtracking :)

Regexes can be evaluated in linear time, so you'd have to send gigabytes worth of text to cause any noticeable slowdown.

I believe with lookahead/behind a regex is technically not "regular" anymore, so you can definitely DoS a regex without GB worth of text.

Sure, but that regex doesn't have lookahead/lookbehind?