如何获取NSString的子字符串？

NSString *haystack = @\"value:hello World:value\";
NSString *haystackPrefix = @\"value:\";
NSString *haystackSuffix = @\":value\";
NSRange needleRange = NSMakeRange(haystackPrefix.length,
                                  haystack.length - haystackPrefix.length - haystackSuffix.length);
NSString *needle = [haystack substringWithRange:needleRange];
NSLog(@\"needle: %@\", needle); // -> \"hello World\"

NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@\"^value:(.+?):value$\" options:0 error:nil];
NSTextCheckingResult *match = [regex firstMatchInString:haystack options:NSAnchoredSearch range:NSMakeRange(0, haystack.length)];
NSRange needleRange = [match rangeAtIndex: 1];
NSString *needle = [haystack substringWithRange:needleRange];

NSString *needle = [haystack componentsSeparatedByString:@\":\"][1];

NSString *myString = @\"abcdefg\";
NSString *mySmallerString = [myString substringToIndex:4];

- (NSString *)getSubstring:(NSString *)value betweenString:(NSString *)separator
{
    NSRange firstInstance = [value rangeOfString:separator];
    NSRange secondInstance = [[value substringFromIndex:firstInstance.location + firstInstance.length] rangeOfString:separator];
    NSRange finalRange = NSMakeRange(firstInstance.location + separator.length, secondInstance.location);

    return [value substringWithRange:finalRange];
}

NSString *myName = [self getSubstring:@\"This is my :name:, woo!!\" betweenString:@\":\"];

NSString *ChkStr = [MyString substringWithRange:NSMakeRange(5, 26)];

NSMakeRange(start, end)

NSMakeRange(start, end- start)

NSString *haystack = @\"MOREvalue:hello World:valueANDMORE\";
NSString *prefix = @\"value:\";
NSString *suffix = @\":value\";
NSRange prefixRange = [haystack rangeOfString:prefix];
NSRange suffixRange = [[haystack substringFromIndex:prefixRange.location+prefixRange.length] rangeOfString:suffix];
NSRange needleRange = NSMakeRange(prefixRange.location+prefix.length, suffixRange.location);
NSString *needle = [haystack substringWithRange:needleRange];
NSLog(@\"needle: %@\", needle);