Just be careful with streams.

https://blog.takipi.com/benchmark-how-java-8-lambdas-and-streams-can-make-your-code-5-times-slower/

damn that is a clever way of doing it. I was thinking there was probably a clever regex solution.....

you don't need both to check if it is smaller than m and bigger than a. It is assumed that all chars are from a-z

I'm curious, does converting it to an array somehow make it more efficient? I just appended to a StringBuilder using a case statement which seems less complicated.