RegexKitLite uses the regular expression engine provided by the libicucore.A.dylib library. However Apple does not officially support linking directly against this library. See this post to the Xcode-users mailing list for details. However despite this there are many iPhone applications available on the App Store than make use of RegexKitLite, so it's unlikely (but not impossible) that Apple will reject your application for making use of it.
