Not the fastest but nice: scalar map { /(M+)/g } ($str);

Compared to the anon monk's benchmark below:

count      3
map        3
regex      3
split      3

          Rate split regex   map count
split 207677/s    --   -7%  -28%  -47%
regex 223050/s    7%    --  -23%  -43%
map   288533/s   39%   29%    --  -27%
count 394162/s   90%   77%   37%    --
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Regards, Karl