How to use JavaScript regex over multiple lines?

DON’T use (.|[\r\n]) instead of . for multiline matching.

DO use [\s\S] instead of . for multiline matching

Also, avoid greediness where not needed by using *? or +? quantifier instead of * or +. This can have a huge performance impact.

See the benchmark I have made: https://jsben.ch/R4Hxu

Using [^]: fastest
Using [\s\S]: 0.83% slower
Using (.|\r|\n): 96% slower
Using (.|[\r\n]): 96% slower

NB: You can also use [^] but it is deprecated in the below comment.

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