Created attachment 403958 [details]
Screenshot of Safari console

Regular expressions sticky flag is not working correctly when used with alternates (|)

Demonstration: (In Safari 13 JS console)

Entering `/a|aa/y.exec("_a")` returns `["a"]` when it should return `null`. This is because matches should be restricted to starting at lastIndex which in this case is index 0.

Chrome, Firefox and Edge all correctly return `null`

Further proof that this is a bug can be seen as follows. Obviously /a|aa/ should be exactly the same as /(a|aa)/ as capturing groups are not relevant. But entering `/(a|aa)/y.exec("_a")` returns `null`.

Similarly wrapping the rhs of the alternate expression shouldn't affect the result but `/a|(aa)/y.exec("_a")` also correctly returns `null`.

This bug seems related to this bug from 2010: https://bugs.webkit.org/show_bug.cgi?id=48102