gistfile1.m

@interface NSString (A)
- (NSString *)stringByReplacingOccurrencesOfString:(NSString *)target options:(NSStringCompareOptions)options range:(NSRange)searchRange withHandler:(NSString * (^)(NSString * occurrence, NSRange rangeOfString))replaceBlock;
@end


@implementation NSString (A)

- (NSString *)stringByReplacingOccurrencesOfString:(NSString *)target options:(NSStringCompareOptions)options range:(NSRange)searchRange withHandler:(NSString * (^)(NSString * occurrence, NSRange rangeOfString))replaceBlock;
{
	NSMutableString * ms = [[self mutableCopy] autorelease];
	NSInteger rangeDifference = 0;
	NSRange foundRange;
	
	do {
		foundRange = [self rangeOfString:target options:options range:searchRange];
		if (foundRange.location == NSNotFound) {
			break;
		}
		
		NSString * foundString = [ms substringWithRange:foundRange];
		NSString * replacement = replaceBlock(foundString, foundRange);
		NSInteger difference = (replacement.length - foundString.length);
		NSRange rangeInMS = NSMakeRange(foundRange.location + rangeDifference, foundRange.length);
		
		[ms replaceCharactersInRange:rangeInMS withString:replacement];
		rangeDifference += difference;
		
		NSInteger searchRangeDelta = NSMaxRange(foundRange) - searchRange.location;
		searchRange.location = NSMaxRange(foundRange);
		searchRange.length -= searchRangeDelta;
		
	} while (searchRange.length > 0);
	
	
	return [[ms copy] autorelease];
}

@end


int main (int argc, const char * argv[])
{
	@autoreleasepool {
		NSString * original = @"The fox ran over the hills and through the woods.";
		NSString * new = [original stringByReplacingOccurrencesOfString:@"the" options:NSCaseInsensitiveSearch range:NSMakeRange(0, original.length)
			withHandler:^(NSString *occurrence, NSRange rangeOfString){
				if ([occurrence isEqual:@"The"]) return @"Theeeee";
				return @"da";
		}];
		
		NSLog(@"%@", new);
	}
	
	return 0;
}