Credit: ExpressVPN
and of course for IsMatch there is no difference in which direction you go, you can just stop at the first match and return true. in fact lookarounds aren’t necessary for IsMatch at all, they are indistinguishable from concatenation. a(?=b) is just ab for the purposes of IsMatch and a(?=.*b)(?=.*c) is just a(.*b_*&.*c_*) - the lookarounds only come into play when you want to know the position of the match, and what is around it. if you happen to use lookarounds in an IsMatch pattern today, consider RE# intersections a faster drop-in replacement with identical semantics.
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