Question

How can I reduce an NSString to lowercase, ascii alpha characters (/a-z/)? Non-english alpha characters should be converted to ascii characters. Specifically, uppercase characters become lowercase (A → a), non-english characters become english characters (ñ → n), and all other characters are discarded.

For example: “Västerås, Sweden” becomes “vasterassweden”.

Answer 1

You can convert a string to an NSData containing bytes representing the string in a different encoding and allow for lossy conversion using dataUsingEncoding:allowLossyConversion. Then you can just convert the NSData back to an NSString:

NSString *unicode = @"Västerås, Sweden";
NSString *ascii = [[NSString alloc]
                   initWithData:[unicode dataUsingEncoding:NSASCIIStringEncoding
                                      allowLossyConversion:YES
                                ]
                       encoding:NSASCIIStringEncoding
                  ];

Result is “Vasteras, Sweden”. Now there is a lowercaseString method etc. if you need to go further.

Edit:

Oops, missed the end of the question “and all other characters are discarded”, you do wish to go further:

NSString *lower = [ascii lowercaseString];
NSString *letters = [[lower
                      componentsSeparatedByCharactersInSet:[[NSCharacterSet letterCharacterSet]
                                                            invertedSet]
                                                           ]
                      componentsJoinedByString:@""
                    ];

That looks horrible (and a good argument to convert to a C-string and do it more directly), but it does the job: componentsSeparatedByCharactersInSet: returns an array of strings by treating the characters in the supplied set as the separators; componentsJoinedByString: joins an array of strings into a single string separated by the supplied string – and we’ve passed an empty joiner.