xstring by Christian Telllechea has been released today. The package provides macros for the manipulation of strings. Some Features:

• tests, if one string is contained in another string (n times),
• extraction of substrings in several ways,
• substitution of some or all occurrences of a substring,
• numerical calculations with string length, position of nth occurence of a substring etc.

You will find the package and its documentation on CTAN.