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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# appending string from variable inside double quotes

Hi I have the following line:
var table = #"<table id=""table_id"" class=""display"">
which is building a table and continues on the next line but I'm just trying to append a string at the end of table_id :
var table = #"<table id=""table_id" + instance + """ class=""display"">
so the final output (if instance = 1234) should be:
<table id="table_id1234" class="display">
But I think the quotes are throwing it off. Any suggestions on how t achieve the last line?
Thanks

A string.Format method placeholder is enough to concatenate instance without cutting through quote signs ({0} is the placeholder):
var table = string.Format(#"<table id=""table_id{0}"" class=""display"">", instance);
Or you can use escape sequence \" for escaping quotes without string literal:
var table = "<table id=\"table_id" + instance + "\" class=\"display\">"
Result:
<table id="table_id1234" class="display">
Demo: .NET Fiddle

Try to use escape character for double quote(\") using this code:
var id = "1234";
var table = "<table id=\"table_id" + id + "\" class=\"display\">";
Here is an online tool for converting string to escape/unescape:
https://www.freeformatter.com/java-dotnet-escape.html
So you can copy the result and place your variables.

I think the best idea and newest idea for this situation is $ sign before your text and with this sign you dont need to extra sign in your string
example
vat table = $"<table id='table_id{instance}' class='display'">

# is used to escape double quotes from one string but in your example, you are actually concatenating three different strings, soyou should escape the third string as well like this:
var table = #"<table id=""table_id" + instance + #" "" class=""display"" >";
Alternatively, you could also use the StringBuilder class which is more memory efficient and might make your strings easier to read.

command line using string.Format() to handle escape characters

Thank for reading my thread.
Here is the command line I like to call within my C# code:
C:\>"D:\fiji-win64\Fiji.app\ImageJ-win64.exe" -eval "run('Bio-Formats','open=D:\\Output\\Untitiled032\\ChanA_0001_0001_0001_0001.tif display_ome-xml')"
This is the exact command I can see from my console window, and it runs and gives me what I need. I want to run this command line from from my C# code, so there is escape character problem I don;t know how to handle
There are two strings I'd like to make them flexible
D:\fiji-win64\Fiji.app\ImageJ-win64.exe
D:\Output\Untitiled032\ChanA_0001_0001_0001_0001.tif
I am wondering how I can use string.Format() to formulate this command line?
This is my current code, it opens the image, but the display_ome-xml did not get called:
string bioformats = "Bio-Formats";
string options = string.Format("open={0} display_ome-xml", fileName.Replace("\\", "\\\\"));
string runCommand = string.Format("run(\"'{0}'\",\"'{1}'\")", bioformats, options);
string fijiCmdText = string.Format("/C \"\"{0}\" -eval {1}", fijiExeFile, runCommand);
where fijiExeFile works fins, it is just the runCommand keeps ignoring the display_ome-xml. Anyone has any suggestions? It is really really confusing. Thanks a lot.

As #Kristian pointed out, # can help here. It also appears that there may be some extra or misplaced \" in the code above. This seems to give the desired output string:
string fijiExeFile = #"D:\fiji-win64\Fiji.app\ImageJ-win64.exe";
string fileName = #"D:\\Output\\Untitiled032\\ChanA_0001_0001_0001_0001.tif";
string bioformats = "Bio-Formats";
string options = string.Format("open={0} display_ome-xml", fileName);
string runCommand = string.Format("run('{0}','{1}')", bioformats, options);
string fijiCmdText = string.Format("\"{0}\" -eval \"{1}\"", fijiExeFile, runCommand);

The easiest way is to use the verbatim string literal.
Just put a # before your string like so:
#"c:\abcd\efgh"
This will disable the backslash escape character
If you need " inside your string, you will have to escape the the quotation marks like so:
#"c:\abcd\efgh.exe ""param1"""
Your example could be:
String.Format(#"""{0}"" -eval ""run('Bio-Formats','open={1} display_ome-xml')""", #"D:\fiji-win64\Fiji.app\ImageJ-win64.exe", #"D:\Output\Untitiled032\ChanA_0001_0001_0001_0001.tif")
or
string p1 = "D:\\fiji-win64\\Fiji.app\\ImageJ-win64.exe";
string p2 = "D:\\Output\\Untitiled032\\ChanA_0001_0001_0001_0001.tif";
String.Format(#"""{0}"" -eval ""run('Bio-Formats','open={1} display_ome-xml')""", p1, p2);

Extract sub-string between two certain words right to left side

Example String
This is an important example about regex for my work.
I can extract important example about regex with this (?<=an).*?(?=for) snippet. Reference
But i would like to extract to string right to left side. According to this question's example; first position must be (for) second position must be (an).
I mean extracting process works back ways.
I tried what i want do as below codes in else İf case, but it doesn't work.
public string FnExtractString(string _QsString, string _QsStart, string _QsEnd, string _QsWay = "LR")
{
if (_QsWay == "LR")
return Regex.Match(_QsString, #"(?<=" + _QsStart + ").*?(?=" + _QsEnd + ")").Value;
else if (_QsWay == "RL")
return Regex.Match(_QsString, #"(?=" + _QsStart + ").*?(<=" + _QsEnd + ")").Value;
else
return _QsString;
}
Thanks in advance.
EDIT
My real example as below
#Var|First String|ID_303#Var|Second String|ID_304#Var|Third String|DI_t55
When i pass two string to my method (for example "|ID_304" and "#Var|") I would like to extract "Second String" but this example is little peace of my real string and my string is changeable.

No need for forward or backward lookahead! You could just:
(.*)\san\s.*\sfor\s
The \s demands whitespace, so you don't match an import*an*t.

One potential problem in your current solution is that the string passed in contains special characters, which needs to be escaped with Regex.Escape before concatenation:
return Regex.Match(_QsString, #"(?<=" + Regex.Escape(_QsStart) + ").*?(?=" + Regex.Escape(_QsEnd) + ")").Value;
For your other requirement of matching RL, I don't understand your requirement.

How do I replace all the spaces with %20 in C#?

I want to make a string into a URL using C#. There must be something in the .NET framework that should help, right?

Another way of doing this is using Uri.EscapeUriString(stringToEscape).

I believe you're looking for HttpServerUtility.UrlEncode.
System.Web.HttpUtility.UrlEncode(string url)

I found useful System.Web.HttpUtility.UrlPathEncode(string str);
It replaces spaces with %20 and not with +.

To properly escape spaces as well as the rest of the special characters, use System.Uri.EscapeDataString(string stringToEscape).

As commented on the approved story, the HttpServerUtility.UrlEncode method replaces spaces with + instead of %20.
Use one of these two methods instead: Uri.EscapeUriString() or Uri.EscapeDataString()
Sample code:
HttpUtility.UrlEncode("https://mywebsite.com/api/get me this file.jpg")
//Output: "https%3a%2f%2fmywebsite.com%2fapi%2fget+me+this+file.jpg"
Uri.EscapeUriString("https://mywebsite.com/api/get me this file.jpg");
//Output: "https://mywebsite.com/api/get%20me%20this%20file.jpg"
Uri.EscapeDataString("https://mywebsite.com/api/get me this file.jpg");
//Output: "https%3A%2F%2Fmywebsite.com%2Fapi%2Fget%20me%20this%20file.jpg"
//When your url has a query string:
Uri.EscapeUriString("https://mywebsite.com/api/get?id=123&name=get me this file.jpg");
//Output: "https://mywebsite.com/api/get?id=123&name=get%20me%20this%20file.jpg"
Uri.EscapeDataString("https://mywebsite.com/api/get?id=123&name=get me this file.jpg");
//Output: "https%3A%2F%2Fmywebsite.com%2Fapi%2Fget%3Fid%3D123%26name%3Dget%20me%20this%20file.jpg"

I needed to do this too, found this question from years ago but question title and text don't quite match up, and using Uri.EscapeDataString or UrlEncode (don't use that one please!) doesn't usually make sense unless we are talking about passing URLs as parameters to other URLs.
(For example, passing a callback URL when doing open ID authentication, Azure AD, etc.)
Hoping this is more pragmatic answer to the question: I want to make a string into a URL using C#, there must be something in the .NET framework that should help, right?
Yes - two functions are helpful for making URL strings in C#
String.Format for formatting the URL
Uri.EscapeDataString for escaping any parameters in the URL
This code
String.Format("https://site/app/?q={0}&redirectUrl={1}",
Uri.EscapeDataString("search for cats"),
Uri.EscapeDataString("https://mysite/myapp/?state=from idp"))
produces this result
https://site/app/?q=search%20for%20cats&redirectUrl=https%3A%2F%2Fmysite%2Fmyapp
Which can be safely copied and pasted into a browser's address bar, or the src attribute of a HTML A tag, or used with curl, or encoded into a QR code, etc.

Use HttpServerUtility.UrlEncode

HttpUtility.UrlDecode works for me:
var str = "name=John%20Doe";
var str2 = HttpUtility.UrlDecode(str);
str2 = "name=John Doe"

HttpUtility.UrlEncode Method (String)

The below code will replace repeating space with a single %20 character.
Example:
Input is:
Code by Hitesh Jain
Output:
Code%20by%20Hitesh%20Jain
Code
static void Main(string[] args)
{
Console.WriteLine("Enter a string");
string str = Console.ReadLine();
string replacedStr = null;
// This loop will repalce all repeat black space in single space
for (int i = 0; i < str.Length - 1; i++)
{
if (!(Convert.ToString(str[i]) == " " &&
Convert.ToString(str[i + 1]) == " "))
{
replacedStr = replacedStr + str[i];
}
}
replacedStr = replacedStr + str[str.Length-1]; // Append last character
replacedStr = replacedStr.Replace(" ", "%20");
Console.WriteLine(replacedStr);
Console.ReadLine();
}

HttpServerUtility.HtmlEncode
From the documentation:
String TestString = "This is a <Test String>.";
String EncodedString = Server.HtmlEncode(TestString);
But this actually encodes HTML, not URLs. Instead use UrlEncode(TestString).