Manipulating Strings(Chapter 6 of Objective-C Phrasebook)

Foundation provides a class for assigning arbitrary attributes to strings. The NSAttributedString class lets you attach dictionaries to ranges in attributed strings. These dictionaries can contain any arbitrary information that you want, including semantic markup. It’s important to remember that sending a -copy message to any NSString subclass will always return an immutable string. If you wanta mutable string, you must use -mutableCopy instead.