Great one-liner. This solution has a space complexity of O(n) because an array is allocated. In place sorting solutions can have a space completity of O(1).

That was my first solution, but it does not consider input validation according to description:
"... accepts a string containing up to 26 unique alphabetical characters."

why did you use regxp to evaluate, simple > would be more elegant!

Its grate, but this is not correct solution, instructions where rather clear you shuld accept max 26 caracters. N

"I have developed an elegant solution to this problem."

1 minute later. . .

"Oh, there was already a method for that."

that make sense ;)

I did the same solution, but in addition I've added a regex to evaluate if we are having up to 26 lowercase chars. I think arg validation is important.

This is why Ruby is beautiful.