Ruby's string slicing functions, now in delicious Objective-C flavor!

slice.m

#import <Foundation/Foundation.h>

@interface NSString (Slicing)
- (unichar)characterAt:(NSInteger)index;
- (NSString *)stringBySlicingFromIndex:(NSInteger)start length:(NSUInteger)length;
- (NSString *)stringBySlicingFromIndex:(NSInteger)start toIndex:(NSInteger)end;
@end

@implementation NSString (Slicing)
- (unichar)characterAt:(NSInteger)index
{
    if (index < 0) index = [self length] - -index;
    return [self characterAtIndex:index];
}

- (NSString *)stringBySlicingFromIndex:(NSInteger)start length:(NSUInteger)length
{
    if (start < 0) start = [self length] - -start;
    NSRange range = NSMakeRange(start, length);
    return [self substringWithRange:range];
}

- (NSString *)stringBySlicingFromIndex:(NSInteger)start toIndex:(NSInteger)end
{
    if (start < 0) start = [self length] - -start;
    if (end < 0) end = [self length] - -end;
    if (start >= [self length] || end >= [self length]) return nil;
    if (start > end) return @"";
    NSUInteger length = end + 1 - start;
    NSRange range = NSMakeRange(start, length);
    return [self substringWithRange:range];
}
@end

int main(int argc, char **argv)
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSString *s = @"hello there";
    NSString *x;
    unichar ch = [s characterAt:-1];
    NSLog(@"%C", ch);
    x = [s stringBySlicingFromIndex:1 length:3];
    NSLog(@"%@", x);
    x = [s stringBySlicingFromIndex:1 toIndex:3];
    NSLog(@"%@", x);
    x = [s stringBySlicingFromIndex:-3 length:2];
    NSLog(@"%@", x);
    x = [s stringBySlicingFromIndex:-4 toIndex:-2];
    NSLog(@"%@", x);
    x = [s stringBySlicingFromIndex:12 toIndex:-1];
    NSLog(@"%@", x);
    x = [s stringBySlicingFromIndex:-2 toIndex:-4];
    NSLog(@"%@", x);
    [pool release];
    return 0;
}