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Splitting the string by few delimeter

I have a string with me and I want to call a rest method from that string. Can anyone think of a better approach then the one I described below for converting str to Rest Call Url.
str : https://P1.P2.com/ProjectName/_build?definitionId=123
Extract P1 and ProjectName and definitionId
Rest Call : https://dev.azure.com/P1/ProjectName/_apis/pipelines/123/runs?api-version=6.0-preview.1
Here, after getting the string, split it with "//" and then split array[1] with "/" and then get P1, ProjectName and then find something with "=" and then the number ?
Not sure if this might be ok.
Update :
I can get this as a URI. But how to convert it then ?

First off, use Uri to parse out the host, path, and query parts of the URI:
var uri = new Uri("https://P1.P2.com/ProjectName/_build?definitionId=123");
For extracting P1 from the host, it's probably going to be easiest to split on .:
string subdomain = uri.Host.Split('.')[0]
For ProjectName, just look at uri.Segments:
string projectName = uri.Segments[1];
For definitionId, parse the query string with HttpUtility.ParseQueryString:
var query = HttpUtility.ParseQueryString(uri.Query);
string definitionId = query["definitionId"];
To construct the new Uri, use a UriBuilder:
var builder = new UriBuilder()
{
Scheme = "https",
Host = "dev.azure.com",
Path = $"{subdomain}/{projectName}_apis/pipelines/{definitionId}/runs",
Query = "api-version=6.0-preview.1",
};
string result = builder.ToString();
See it on dotnetfiddle.net

Insert value into String after the last slash

I need to insert some string value after the last slash. I have such string value:
string url = "http://blog.loc/blog/news/sport/slug1_slug2_slug3-slug";
I need to get this value:
"http://blog.loc/blog/news/sport/hot_slug1_slug2_slug3-slug"
So, I need to insert hot_ (for example), after the last slash. Could anyone help me?

I know you asked for regex, but it's not really necessary in my opinion.
You can just use string.Insert:
string url = "http://blog.loc/blog/news/sport/slug1_slug2_slug3-slug";
url = url.Insert(url.LastIndexOf("/") + 1, "hot_");
url now holds the value: http://blog.loc/blog/news/sport/hot_slug1_slug2_slug3-slug

Regex Method :
string url = "http://blog.loc/blog/news/sport/slug1_slug2_slug3-slug";
var matches = Regex.Matches(url, "/");
var match = matches[matches.Count - 1];
string result = url.Insert(match.Index + 1, "hot_")

unable to read querystring in asp.net

This is my url which contains 4 querystrings(desc,url,img,title).
http://localhost:4385/Default?desc=Home%20Page&url=http://localhost:4385/&img=http://localhost:4385/images/ribbon-img.png&title=
I read querystrings like below,
string title = Request.QueryString["desc"];
string pageurl = Request.QueryString["url"];
string alttext = Request.QueryString["title"];
string imageurl = Request.QueryString["img"];
The output i get is:
title=Home Page&url=http://localhost:4385/&img=http://localhost:4385/images/ribbon-img.png&title="
it takes entire url to first querstring, this is not my expected output.
I expect values to all querystring variables
can anyone please help me

The URL format is incorrect i feel, because the slash / character will be sent as %2F in the query string but that was not done in your URL format.
Update:
Respose.Redirect("http://localhost:4385/Default?desc=Home%20Page&url="+Uri.EscapeDataString("http://localhost:4385/")+"&img="+Uri.EscapeDataString("http://localhost:4385/images/ribbon-img.png")+"&title=");

The problem is that you are not creating the QueryString with proper encoding. .NET framework has HttpUtility.ParseQueryString Method to simplify this problem of encoding. Try this code
//are you sure your URL doesn't have an ".aspx" extension?
var url = " http://localhost:4385/Default.aspx?";
var queryString = System.Web.HttpUtility.ParseQueryString(string.Empty);
queryString["desc"] = "Home Page";
queryString["url"] = "http://localhost:4385/";
queryString["image"] = "http://localhost:4385/images/ribbon-img.png";
queryString["title"] = "";
Response.Redirect(url + queryString.ToString());
Now the QueryString will look like this.
var urlWithQueryString = " http://localhost:4385/Default.aspx?desc=Home+Page&url=http%3a%2f%2flocalhost%3a4385%2f&image=http%3a%2f%2flocalhost%3a4385%2fimages%2fribbon-img.png&title="
Now parsing can be done using the method you tried
string title = Request.QueryString["desc"];
string pageurl = Request.QueryString["url"];
string alttext = Request.QueryString["title"];
string imageurl = Request.QueryString["image"]; //you have wrongly typed "img" here

How to delete string from another string position in c#

I have two string:
string url = HttpContext.Current.Request.Url.AbsoluteUri;
//give me :
//url = http://localhost:1302/TESTERS/Default6.aspx?tabindex=2&tabid=15
And:
string path = HttpContext.Current.Request.Url.AbsolutePath;
//give me:
//path = /TESTERS/Default6.aspx
Now I want to get the string:
http://localhost:1302
So what I am thinking of is I will find the position of path in url and remove the sub-string from this position in url.
What I tried:
string strApp = url.Remove(url.First(path));
or
string strApp = url.Remove(url.find_first_of(path));
but I can't find the write way to express this idea. How can I archive my goal?

So basically you want the URL, from the start up to the beginning of your path.
You don't need to "remove" that part, only take characters up to that precise point. First, you can get that location with a simple IndexOf as it returns the position of the first character that matches your string. After this, simply take the part of url that goes from 0 to that index with Substring.
string url = "http://localhost:1302/TESTERS/Default6.aspx?tabindex=2&tabid=15";
string path = "/TESTERS/Default6.aspx";
int indexOfPath = url.IndexOf(path);
string strApp = url.Substring(0, indexOfPath); // gives http://localhost:1302
Which you can shorten to
string strApp = url.Substring(0, url.IndexOf(path));

you can also do something like below code to get the Host of URI
Uri uri =HttpContext.Current.Request.Url.AbsoluteUri ;
string host = uri.Authority; // "example.com"

Here is another option.. this doesn't require any string manipulation:
new Uri(HttpContext.Current.Request.Url, "/").AbsoluteUri
It generates a new Uri which is the path "/" relative to the original Url

You should just use this instead:
string baseURL = HttpContext.Current.Context.Request.Url.Scheme + "://" +
HttpContext.Current.Context.Request.Url.Authority;

This should not be solved using string manipulation. HttpContext.Current.Request.Url returns an Uri object which has capabilities to return the information you request.
var requestUrl = HttpContext.Current.Request.Url;
var result = requestUrl.GetComponents(UriComponents.SchemeAndServer,
UriFormat.Unescaped);
// result = "http://localhost:1302"

c# regex.ismatch using a variable

I have the following code which works fine but i need to replace the site address with a variable...
string url = HttpContext.Current.Request.Url.AbsoluteUri; // Get the URL
bool match = Regex.IsMatch(url, #"(^|\s)http://www.mywebsite.co.uk/index.aspx(\s|$)");
I have tried the following but it doesn't work, any ideas???
string url = HttpContext.Current.Request.Url.AbsoluteUri; // Get the URL
string myurl = "http://www.mywebsite.co.uk/index.aspx";
bool match = Regex.IsMatch(url, #"(^|\s)"+myurl+"(\s|$)");

You are missing a #:
bool match = Regex.IsMatch(url, #"(^|\s)" + myurl + #"(\s|$)");
The reason that you need the extra # is because the # applies only to the string literal immediately following it. It does not apply to the entire rest of the line.
You should also consider escaping your URL:
bool match = Regex.IsMatch(url, #"(^|\s)" + Regex.Escape(myurl) + #"(\s|$)");