The problem is not just for partition, but also involves split and scan.
I think your regex /^=*/ is unnecessarily complex. Your point can be made by /\A/, which is simpler.
I tried with four regex patterns /\A/, /\A.*/, /\z/, /.*\z/, and compared methods split, partition, scan. The result of the first example in each group below matches the second and the third, and the fourth one matches the middle element. So far, so good.
"foo".match(/\z/).then{[_1.pre_match, _1[0], _1.post_match]} # => ["foo", "", ""]
"foo".split(/(\z)/, -1) # => ["foo", "", ""]
"foo".partition(/\z/) # => ["foo", "", ""]
"foo".scan(/\z/) # => [""]
"foo".match(/\A.*/).then{[_1.pre_match, _1[0], _1.post_match]} # => ["", "foo", ""]
"foo".split(/(\A.*)/, -1) # => ["", "foo", ""]
"foo".partition(/\A.*/) # => ["", "foo", ""]
"foo".scan(/\A.*/) # => ["foo"]
In the following, we see inconsistency:
"foo".match(/\A/).then{[_1.pre_match, _1[0], _1.post_match]} # => ["", "", "foo"]
"foo".split(/(\A)/, -1) # => ["foo"]
"foo".partition(/\A/) # => ["foo", "", ""]
"foo".scan(/\A/) # => [""]
"foo".match(/.*\z/).then{[_1.pre_match, _1[0], _1.post_match]} # => ["", "foo", ""]
"foo".split(/(.*\z)/, -1) # => ["", "foo", ""]
"foo".partition(/.*\z/) # => ["", "foo", ""]
"foo".scan(/.*\z/) # => ["foo", ""]
The problematic cases and their expected values (in terms of consistency) are:
"foo".split(/(\A)/, -1) # => ["foo"], expected [ "", "", "foo"]
"foo".partition(/\A/) # => ["foo", "", ""], expected ["", "", "foo"]
"foo".scan(/.*\z/) # => ["foo", ""], expected ["foo"]
The case described in the issue is the second case above.
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