Inconsistency between engines when using reluctant quantifier in negative look ahead

Question

I found something odd when using a reluctant quantifier in a negative look ahead.

When creating a regex to assert a maximum of 3 uppercase characters, I devised this:

^(?!(.*?[A-Z]){4}).*$

which works on rubular, but not on regex101.

Why is that?

Answer 1

^, $ matches beginning/end of line in Ruby.

While in another languages, ^, $ matches the beginning/end of the string unless multiline mode (m) is specified. (Some regular expression engine requires g flag to match multiple times.)