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Good point. Neither old nor new (which matches regex) behaviors conform the documentation: "Empty matches are included in the result unless they touch the beginning of another match." It is easy to exclude empty matches that touch the *ending* of another match. This would be consistent with the new behavior of split() and sub().
But this would break a one existing test for issue817234. Though that issue shouldn't rely on this detail. The test should just test that iterating doesn't hang.
And this would break a regular expression in pprint.
PR 4678 implements this version. I don't know what version is better.
>>> list(re.finditer(r"\b|:+", "a::bc"))
[<re.Match object; span=(0, 0), match=''>, <re.Match object; span=(1, 1), match=''>, <re.Match object; span=(1, 3), match='::'>, <re.Match object; span=(5, 5), match=''>]
>>> re.sub(r"(\b|:+)", r"[1円]", "a::bc")
'[]a[][::]bc[]'
With PR 4471 the result of re.sub() is the same, but the result of re.finditer() is as in msg307424.