去除 username中的空格，table newline，nextline 
代码如下：

NSCharacterSet *whitespace = [NSCharacterSet  whitespaceAndNewlineCharacterSet];

NSString * string =@"liujun        \t          ";

string = [string  stringByTrimmingCharactersInSet:whitespace]; 

注释：

stringByTrimmingCharactersInSet: 
Returns a new string made by removing from both ends of the receiver characters contained in a given character set.

whitespaceAndNewlineCharacterSet 
Returns a character set containing only the whitespace characters space (U+0020) and tab (U+0009) and the newline and nextline characters (U+000A–U+000D, U+0085).

另外可以用 whitespaceCharacterSet 替换 whitespaceAndNewlineCharacterSet 区别newline nextline 
whitespaceCharacterSet 
Returns a character set containing only the in-line whitespace characters space (U+0020) and tab (U+0009).

NSString *content = @"    kahdahdah       \t           ";

NSString *temptext = [messageTextField.text stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
NSString *text = [temptext stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet ]];

第1行是去除2端的空格

第2行是去除回车

NSCharacterSet的各个枚举类型的含义如下:

 1 controlCharacterSet//控制符

 2 whitespaceCharacterSet

 3 whitespaceAndNewlineCharacterSet//空格换行

 4 decimalDigitCharacterSet//小数

 5 letterCharacterSet//文字

 6 lowercaseLetterCharacterSet//小写字母

 7 uppercaseLetterCharacterSet//大写字母

 8 nonBaseCharacterSet//非基础

 9 alphanumericCharacterSet//字母数字

10 decomposableCharacterSet//可分解

11 illegalCharacterSet//非法

12 punctuationCharacterSet//标点

13 capitalizedLetterCharacterSet//大写

14 symbolCharacterSet//符号

15 newlineCharacterSet//换行符