D'oh! Sorry. I read your first post but somehow failed to read the very nice description of the app. Brain damage I guess.
So this is pretty easy from a cross platform point of view. Just about any language (except C or C++) has decent enough string processing to do what you want. For the Ultimate in cross platform look at something like REXX or Perl. If you want to compile it you could use Free Pascal. If you want to learn a trendy Web oriented language you could use Python or Ruby.
The biggest problem you'll face with something like Java, is wading your way through the various versions of Java and the many possible JVM's. Jave is not my favorite

Pascal was supposed to be the next logical step for BASIC programmers. Of course that was in the 80's

Have fun. No matter what you choose, it should be fun.