Sample functions for String Parsers in Lisp

[Nordlöw]

Has anybody any good sample code in elisp that shows how to
conveniently do parsing of code contained in a string?

Reason: I am writing a parser that converts Emacs regexps (back) into
rx expressions. I need this for my file magic pattern matcher in order
to read as little data as possible from the file (locality) and do
several matchings in parallel.

I think the main loop should be string-iteration-driven using while()
and aref() and length() I presume.

Any suggestions?,
Nordlöw