[plt-scheme] Characters in a String

Other people have already mentioned "string->list" and a tokenizer
procedure, both of which sound like they'd work for your immediate
problem.

Other methods I find helpful in reading from strings are string ports
(search for "open-input-string" in the Help Desk) and "string-ref"
(especially when I want random access, and when I might only need to
access a small percentage of the characters in a large string, and
when I'm outsmarting myself by trying to make life easier for the GC).