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Would you be able to come up with an example that would be
useful for that kind of construction?

"(a*){4}" will always match "a" as many times as possible,
and than match the empty string 3 more times. So it has the
effect of "a*", but in addition will kill the grouping
effect since the given group will always be empty. With that
in mind considering it as a syntax error seems correct.

Do you agree?