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trim string before character but still keep the remain part after it

So I have this string which I have to trim and manipulate a little with it.
My string example:
string test = "studentName_123.pdf";
Now, what I want to do is somehow extract only the _123 part and at the end I need to have studentName.pdf
What I have tried:
string test_extracted = test.Substring(0, test.LastIndexOf("_") )+".pdf";
This also works but the thing is that I don't want to add the ".pdf" suffix at the end of the string manually because I can have strings that are not pdf, for ex. studentName.docx , studentName.png.
So basically I just want the "_123" part removed but still keep the remain part after that.

I think this might help you:
string test = "studentName_123.pdf";
string test_extracted = test.Substring(0, test.LastIndexOf("_") )+ test.Substring(test.LastIndexOf("."),test.Length - test.LastIndexOf(".") );
Using Remove(int startIndex, int count):
string test = "studentName_123.pdf";
string test_extracted = test.Remove(test.LastIndexOf("_"), test.LastIndexOf(".") - test.LastIndexOf("_"));

Sounds like you mean something like this?
string extension = Path.GetExtension(test);
string pdfName = Path.GetFileNameWithoutExtension(test).Split('_')[0];
string fullName = pdfName + extension;

Since you know what value you will always be replacing in your strings, "_123", to base on your example, just utilize the replace method and replace it with nothing since the method expects two arguments;
string test_extracted = test.replace('_123', '');

This could be solved with a regular expression like this
(\w*)_.*(\.\w*) where the first capture group (\w*) matches everything before the underscore and the second group (\.\w*) matches the file extensions.
Lastly we just have to concat the groups without the stuff inbetween like so:
string test = "studentName_123.pdf";
var regex = Regex.Match(test, #"(\w*)_.*(\.\w*)");
string newString = regex.Groups[1].Value + regex.Groups[2].Value;

C# Finding numerical value from specific string

I have a string of varying length that I am trying to retrieve a number from. The format of the string is always:
"some text lines
FC = 1234
more text here
and so on"
So I know the string of numbers comes after "FC = ", and I know it finishes at the next \n. How can I return this number (which will vary in size) into a new string?

Try the following code snippet:
var str = "some text lines \nFC = 1234\n more text here and so on";
Console.WriteLine(Regex.Match(str, #"\d+\.*\d*").Value);

Thanks to all. Think I managed to find a way with Regex, based on ScareCrow's suggestion:
string rgSearch = searchString + #"\d+\.*\d*";
FC = Regex.Match(diagnostics, rgSearch).Value;
FC = FC.Replace(searchString, ""); //Leaves the number only

Remove part of a string between an start and end

Code first:
string myString = "<at>onePossibleName</at> some question here regarding <at>disPossibleName</at>"
// some code to handle myString and save it in myEditedString
Console.WriteLine(myEditedString);
//output now is: some question here regarding <at>disPossibleName</at>
I want to remove <at>onePossibleName</at> from myString. The string onePossibleName and disPossbileName could be any other string.
So far I am working with
string myEditedString = string.Join(" ", myString.Split(' ').Skip(1));
The problem here would be that if onePossibleName becomes one Possible Name.
Same goes for the try with myString.Remove(startIndex, count) - this is not the solution.

There will be different method depending on what you want, you can go with a IndexOf and a SubString, regex would be a solution too.
// SubString and IndexOf method
// Usefull if you don't care of the word in the at tag, and you want to remove the first at tag
if (myString.Contains("</at>"))
{
var myEditedString = myString.Substring(myString.IndexOf("</at>") + 5);
}
// Regex method
var stringToRemove = "onePossibleName";
var rgx = new Regex($"<at>{stringToRemove}</at>");
var myEditedString = rgx.Replace(myString, string.Empty, 1); // The 1 precise that only the first occurrence will be replaced

You could use this generic regular expression.
var myString = "<at>onePossibleName</at> some question here regarding <at>disPossibleName</at>";
var rg = new Regex(#"<at>(.*?)<\/at>");
var result = rg.Replace(myString, "").Trim();
This would remove all 'at' tags and the content between. The Trim() call is to remove any white space at the beginning/end of the string after the replacement.

string myString = "<at>onePossibleName</at> some question here regarding <at>disPossibleName</at>"
int sFrom = myString.IndexOf("<at>") + "<at>".Length;
int sTo = myString.IndexOf("</at>");
string myEditedString = myString.SubString(sFrom, sFrom - sTo);
Console.WriteLine(myEditedString);
//output now is: some question here regarding <at>disPossibleName</at>

Identify the string that does not exists in another string using regex and C#

I am trying to capture a string that does not contains in another string.
string searchedString = " This is my search string";
string subsetofSearchedString = "This is my";
My output should be "Search string". I would like to go with only regex so that I can handle complex strings.
The below is the code that I have tried so far and I am not successful.
Match match = new Regex(subsetofSearchedString ).Match(searchedString );
if (!string.IsNullOrWhiteSpace(match.Value))
{
UnmatchedString= UnmatchedString.Replace(match.Value, string.Empty);
}
Update : The above code is not working for the below texts.
text1 = 'Property Damage (2015 ACURA)' Exposure Added Automatically for IP:Claimant DriverLoss Reserve Line :Property DamageReserve Amount $ : STATIP Role(s): Owner, DriverExposure Owner :Jaimee Watson_csr Author:
text2 = 'Property Damage (2015 ACURA)' Exposure Added Automatically for IP:Claimant DriverLoss Reserve Line :Property DamageReserve Amount $ : STATIP Role(s): Owner, Driver
Match match = new Regex(text2).Match(text1);

You can use Regex.Split:
var ans = Regex.Split(searchedString, subsetofSearchedString);
If you want the answer as a single string minus the subset, you can join it:
var ansjoined = String.Join("", ans);
Replacing with String.Empty will also work:
var ans = Regex.Replace(searchedString, subsetOfSearchedString, String.Empty);

Answer :
Regex wasn't working for me because of the presence of metacharacters in my string. Regex.Escape did not help me with the comparison.
String Contains worked like a charm here
if (text1.Contains(text2))
{
status = TestResult.Pass;
text1= text1.Replace(text2, string.Empty);
}

Replacing 2 new lines and tab with string using regular expression

This seems really simple, but I have a string that I want to replace a string with a tab and 2 new lines and it isn't working.
string newString = "\tMyVariable : Bool\n\t\nEND_VAR";
string pattern = "\n\t\nEND_VAR";
string original = "VAR_GLOBAL\n\t\nEND_VAR\n";
string updatedString = Regex.Replace(original,pattern,newString);
updatedString never gets updated, it remains at "VAR_GLOBAL\n\t\nEND_VAR\n", where it should be "VAR_GLOBAL\tMyVariable : Bool\n\t\nEND_VAR\n". I'm not sure why it won't change.

While Regex might not be the best suited for such scenario, please find below a sample code which will suit your needs (Regex test).
string newString = "\tMyVariable : Bool\n\t\nEND_VAR";
string pattern = "\\n\\t\\nEND_VAR";
string original = "VAR_GLOBAL\n\t\nEND_VAR\n";
string updatedString = Regex.Replace(original,pattern,newString);
The other (maybe simplier) option would be to do a straight replace like in:
string newString = "\tMyVariable : Bool\n\t\nEND_VAR";
string updatedString = newString.Replace("\n\t\n", "\t");