1.常用功能使用
1）初始化
NSString *astring = @"This is a String!"; //固定字符串NSString *str = [NSString stringWithFormat:@"asd%d%@",123,@"ttt"]; //拼组字符串

2）截取
NSString *str = [@"asd" substringFromIndex:1]; //从第几位开始截到最后NSString *str1 = [str substringToIndex:1]; //从开始位置截到第几位NSString *str = [[@"asd" substringFromIndex:1] substringToIndex:1]; //从第几位开始截几个字符

3）分解
NSArray *array = [@"asd|tes|mmm" componentsSeparatedByString:@"|"];

4）查找
NSRange range = [@"12rasd124" rangeOfString:@"asd"]; if (range.location != NSNotFound) { NSString *str = [@"12rasd124" substringWithRange:range]; }

5）替换
NSString *str = [@"asdasd" stringByReplacingOccurrencesOfString:@"asd" withString:@"ok"];

6）路径相关
[filepath lastPathComponent]; //获得文件名[filepath pathExtension]; //获得文件扩展名[filtpaht stringByDeletingLastPathComponent]; //获得文件的所在文件夹路径[filtpath stringByDeletingPathExtension]; //获得去除扩展名的文件路径

7）字符串是否为空
if (str.lenght > 0) //不管str是不是nil都可以这么判断

8）字符串比较
if ([@"asd" isEqualToString:@"asd"]) { }

9）字符串比大小
NSString *stra = @"asd"; NSString *strb = @"dfs"; NSComparisonResult result = [stra compare:strb]; if (result == NSOrderedSame) { } else if (result == NSOrderedAscending) { } else { }

2.常用扩展功能(类别扩展)

1）md5

• (NSString )md5HexDigest
  {
  const char str = [self UTF8String];
  unsigned char result[CC_MD5_DIGEST_LENGTH];
  CC_MD5(str, strlen(str), result);
  NSMutableString *ret = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH];

  for(int i = 0; i<CC_MD5_DIGEST_LENGTH; i++) {
  [ret appendFormat:@"%02X",result[i]];
  }
  return ret;

}
2）urlEncode

• (NSString *)stringEncode {
  NSString *encodedString = (NSString )CFBridgingRelease(CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (CFStringRef)self, NULL, CFSTR(":/?#[]@!$ &'()+,;="<>%{}|\^~`"), kCFStringEncodingUTF8));
  return encodedString;
  }

• (NSString *)stringDecode {
  NSString *result = [(NSString *)self stringByReplacingOccurrencesOfString:@"+" withString:@" "];
  result = [result stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
  return result;
  }
  3）sizeFromFont

• (CGSize)boundingRectWithSize:(CGSize)size
  withTextFont:(UIFont *)font {
  if ([self isEqualToString:@""]) {
  return CGSizeMake(0, 0);
  }
  NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];
  paragraphStyle.lineBreakMode = NSLineBreakByWordWrapping;
  paragraphStyle.lineSpacing = 0;
  NSDictionary *attributes = @{NSFontAttributeName:font, NSParagraphStyleAttributeName:paragraphStyle.copy};

  return [self boundingRectWithSize:size options:NSStringDrawingUsesLineFragmentOrigin attributes:attributes context:nil].size;
  }

4. 是否是邮箱格式

• (BOOL)isEmail
  {
  NSString emailRegEx =
  @"(?:[a-z0-9!#$%\&'+/=?\^_{|}~-]+(?:\\.[a-z0-9!#$%\\&'*+/=?\\^_{|}"
  @"~-]+)|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\"
  @"x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])")@(?:(?:[a-z0-9](?:[a-"
  @"z0-9-][a-z0-9])?\.)+a-z0-9?|\[(?:(?:25[0-5"
  @"]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-"
  @"9][0-9]?|[a-z0-9-][a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21"
  @"-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])";

  NSPredicate *regExPredicate = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegEx];
  return [regExPredicate evaluateWithObject:[self lowercaseString]];
  }

5. Base64

• (NSString *)base64 {
  // Create NSData object
  NSData *nsdata = [self dataUsingEncoding:NSUTF8StringEncoding];

  // Get NSString from NSData object in Base64
  NSString *base64Encoded = [nsdata base64EncodedStringWithOptions:0];

  // Print the Base64 encoded string
  return base64Encoded;
  }

• (NSString *)base64Decoded {
  // NSData from the Base64 encoded str
  NSData *nsdataFromBase64String = [[NSData alloc] initWithBase64EncodedString:self options:0];

  // Decoded NSString from the NSData
  NSString *base64Decoded = [[NSString alloc] initWithData:nsdataFromBase64String encoding:NSUTF8StringEncoding];

  return base64Decoded;
  }