convert camel case to/from snake case in Objective-C.

CamelCase.m

#import <Foundation/Foundation.h>

NSString *toCamelCase(NSString *s)
{
    return [[[s capitalizedString] componentsSeparatedByCharactersInSet:[[NSCharacterSet characterSetWithCharactersInString:@"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"] invertedSet]] componentsJoinedByString:@""];
}

NSString *splitOnCapital(NSString *s) {
    NSRange upcaseRange = NSMakeRange('A', 26);
    NSRange numberRange = NSMakeRange('0', 10);
    NSIndexSet *upcaseSet = [NSIndexSet indexSetWithIndexesInRange:upcaseRange];
    NSIndexSet *numberSet = [NSIndexSet indexSetWithIndexesInRange:numberRange];
    NSMutableString *result = [NSMutableString string];
    NSMutableString *oneWord = [NSMutableString string];
    BOOL prevLetterIsUc = NO;
    for (int i = 0; i < s.length; i++) {
        char oneChar = [s characterAtIndex:i];
        if ([upcaseSet containsIndex:oneChar]||[numberSet containsIndex:oneChar]) {
            if ( prevLetterIsUc ) {
            } else {
                if (result.length == 0) {
                    [result appendFormat:@"%@", [oneWord lowercaseString]];
                }else {
                    [result appendFormat:@"_%@", [oneWord lowercaseString]];
                }
                oneWord = [NSMutableString string];
            }
            prevLetterIsUc = YES;
        } else {
            prevLetterIsUc = NO;
        }
        [oneWord appendFormat:@"%c", oneChar];
    }
    if (oneWord.length > 0) {
        [result appendFormat:@"_%@", [oneWord lowercaseString]];
    }
    return result;
}

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        NSLog(@"%@", toCamelCase(@"test Example one"));
        NSLog(@"%@", toCamelCase(@"example Two "));
        NSLog(@"%@", toCamelCase(@"is_this_3Rd_EXAMPLE?.."));
        NSLog(@"%@", splitOnCapital(@"TestExampleOne"));
        NSLog(@"%@", splitOnCapital(@"ExampleTwo "));
        NSLog(@"%@", splitOnCapital(@"IsThis3RdEXAMPLE?.."));
    }
    return 0;
}