using System.Text.RegularExpressions;
public static void Main()
var input = "=inbound&CallerState=IN&ToZip=&CallSid=CA35b8f8162eeffebf07d4fc7c9aeadd94&To=%2B123475&CallerZip=406&ToCountry=US&ApiVersion=2010-04-01&CalledZip=&CalledCity=&CallStatus=ringing&From=%2B12606&AccountSid=ACc6d06b4cb61ccbfa61bf461957a5a626&CalledCountry=US&CallerCity=AUBURN&Caller=%2B1265506&FromCountry=US&ToCity=&FromCity=AURN&CalledState=IN&FromZip=466&FromState=IN";
var regex = new Regex(@"\&CallSid=([a-z0-9]+)\&", RegexOptions.IgnoreCase | RegexOptions.Compiled);
var match = regex.Match(input);
Console.WriteLine(result);