//OC获取中间的字符串
NSString *middleStr; 
//获取中间的字符串
NSString *frontStr = @"id"; 
//从该字符开始
NSString *behindStr = @"?"; 
//到该字符结束
NSRange range;
range.location = [string rangeOfString:frontStr].location + frontStr.length;
range.length = [string rangeOfString:behindStr].location - range.location;
middleStr = [NSString stringWithFormat:@"%@", [string substringWithRange:range]];
NSLog(@"%@",middleStr);