Great, clean solution!

Use single quote.

I... wish I knew when recursion would be useful. My solution separated the numbers into arrays by 10^n places and then discovered for 9s, 5-8s, 4s, 1-3s. I am an absolute beginner so I will forgive myself but thank you for your kind submission. Helps people like me learn so much (including when recursion would be useful...), and so much quicker. I'm sure I speak for many.

Much appreciated.

this is brilliant. love the recursion.

Could you/someone explain how this works?

"Hashes enumerate their values in the order that the corresponding keys were inserted."

... I love this language

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."

....I love this language

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."

This solution is similar to mine (but I used looping)