CamelCase to underscores and back in Objective-C

Question

I'm looking for a simple, efficient way to convert strings in CamelCase to underscore notation (i.e., MyClassName -> my_class_name) and back again in Objective C.

My current solution involves lots of rangeOfString, characterAtIndex, and replaceCharactersInRange operations on NSMutableStrings, and is just plain ugly as hell :) It seems that there must be a better solution, but I'm not sure what it is.

I'd rather not import a regex library just for this one use case, though that is an option if all else fails.

Answer 1

If your concern is just the visibility of your code, you could make a category for NSString using the methods you've designed already. That way, you only see the ugly mess once. ;)

For instance:

@interface NSString(Conversions) {
     - (NSString *)asCamelCase;
     - (NSString *)asUnderscored;
}

@implementation NSString(Conversions) {
     - (NSString *)asCamelCase {
          // whatever you came up with
     }
     - (NSString *)asUnderscored {
          // whatever you came up with
     }
}

EDIT: After a quick Google search, I couldn't find any way of doing this, even in plain C. However, I did find a framework that could be useful. It's called RegexKitLite. It uses the built-in ICU library, so it only adds about 20K to the final binary.

Answer 2

Chris's suggestion of RegexKitLite is good. It's an excellent toolkit, but this could be done pretty easily with NSScanner. Use -scanCharactersFromSet:intoString: alternating between +uppercaseLetterCharacterSet and +lowercaseLetterCharacterSet. For going back, you'd use -scanUpToCharactersFromSet: instead, using a character set with just an underscore in it.

Answer 3

How about these:

NSString *MyCamelCaseToUnderscores(NSString *input) {
    NSMutableString *output = [NSMutableString string];
    NSCharacterSet *uppercase = [NSCharacterSet uppercaseLetterCharacterSet];
    for (NSInteger idx = 0; idx < [input length]; idx += 1) {
        unichar c = [input characterAtIndex:idx];
        if ([uppercase characterIsMember:c]) {
            [output appendFormat:@"_%@", [[NSString stringWithCharacters:&c length:1] lowercaseString]];
        } else {
            [output appendFormat:@"%C", c];
        }
    }
    return output;
}

NSString *MyUnderscoresToCamelCase(NSString *underscores) {
    NSMutableString *output = [NSMutableString string];
    BOOL makeNextCharacterUpperCase = NO;
    for (NSInteger idx = 0; idx < [underscores length]; idx += 1) {
        unichar c = [underscores characterAtIndex:idx];
        if (c == '_') {
            makeNextCharacterUpperCase = YES;
        } else if (makeNextCharacterUpperCase) {
            [output appendString:[[NSString stringWithCharacters:&c length:1] uppercaseString]];
            makeNextCharacterUpperCase = NO;
        } else {
            [output appendFormat:@"%C", c];
        }
    }
    return output;
}

Some drawbacks are that they do use temporary strings to convert between upper and lower case, and they don't have any logic for acronyms, so myURL will result in my_u_r_l.