We can do this by splitting and then joining. Requires OS X 10.5+ for the componentsSeparatedByCharactersInSet:
NSCharacterSet *charactersToRemove = [[NSCharacterSet alphanumericCharacterSet] invertedSet];
NSString *strippedReplacement = [[someString componentsSeparatedByCharactersInSet:charactersToRemove] componentsJoinedByString:@""];
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