Write a function regroup that receives as argument a string consisting of mixed-case letters, and returns a regrouping of the characters in the string. The regrouping is as follows: in the returned string, first there come the lower-case letters, in the same order as they came in the argument string; and next come the upper-case letters, in the same order as they came in the argument string.
>>> regroup('rTbS') rbTS >>> regroup('aDFtrZXpw') atrpwDFZX >>> regroup('allarelower') allarelower >>> regroup('ALLAREUPPER') ALLAREUPPER