You may want to try out the following regex. Regex101 link
((?:\?.*?&|\?)journey=)[^&]*
Try out the following code to replace the value of journey to replacement
string url = "http://www.whitelabelhosting.co.uk/flight-search.php?dept=any&journey=R&DepTime=0900";
string newUrl = Regex.Replace(url, @"((?:\?.*?&|\?)journey=)[^&]*", "$1"+"replacement");
Remember to add the following to your file:
using System.Text.RegularExpressions;
You can do the same for DepTime
using the following regex:
((?:\?.*?&|\?)DepTime=)[^&]*
1
solved replace the matched expression from string and its next charecter?POST operation