Develop Reference

How do I escape a backslash in the following regex c#

Here is my function, I'm trying to replace a string in a file, but c# tells me my regex is malformed. Any ideas?
public void function(string fileName, string path) {
string pathToAmmend = #"$SERVERROOT\pathpath";
string newPath = #"$SERVERROOT\" + path;
File.WriteAllText(fileName, Regex.Replace(File.ReadAllText(fileName), pathToAmmend, newPath));
....
}
It works if i change the strings to:
string pathToAmmend = #"$SERVERROOT\\pathpath";
string newPath = #"$SERVERROOT\\" + path;
But then I have two slashes and I only want one slash.

It sounds like you don't actually need a regular expression at all. It sounds like you quite possibly just want string.Replace:
// Split into three statements for clarity.
string input = File.ReadAllText(fileName);
string output = input.Replace(pathToAmend, newPath);
File.WriteAllText(output);
Only use regular expressions when you're genuinely trying to match patterns.

A \ is a special escaping character in regular expressions. You have to escape it so that it will be interpreted as a literal \ and not an escape sequence. $ is also a special character (an end anchor), so you'll want to escape that as well.
string pathToAmmend = #"\$SERVERROOT\\pathpath";
Using # to create a verbatim string only means you don't have to escape the \ for the sake of the C# compiler. You still have escape the \ for in a regular expression pattern. Without the verbatim string this would be:
string pathToAmmend = "\\$SERVERROOT\\\\pathpath";
Of course, as Jon Skeet points out, for something this simple, regular expressions aren't really the best way to go here.

Replacing double backwards slashes with single ones in c#

I need to replace double quotes with single so that something like this
\\\\servername\\dir1\\subdir1\\
becomes
\\servername\dir1\subdir1\
I tried this
string dir = "\\\\servername\\dir1\\subdir1\\";
string s = dir.Replace(#"\\", #"\");
The result I get is
\\servername\\dir1\\subdir1\\
Any ideas?

You don't need to replace anything here. The backslashes are escaped, that's why they are doubled.
Just like \t represents a tabulator, \\ represents a single \. You can see the full list of Escape Sequences on MSDN.
string dir = "\\\\servername\\dir1\\subdir1\\";
Console.WriteLine(dir);
This will output \\servername\dir1\subdir1\.
BTW: You can use the verbatim string to make it more readable:
string dir = #"\\servername\dir1\subdir1\";

There is no problem with the code for replacing. The result that you get is:
\servername\dir1\subdir1\
When you are looking at the result in the debugger, it's shown as it would be written as a literal string, so a backslash characters is shown as two backslash characters.
The string that you create isn't what you think it is. This code:
string dir = "\\\\servername\\dir1\\subdir1\\";
produces a string containing:
\\servername\dir1\subdir1\
The replacement code does replace the \\ at the beginning of the string.
If you want to produce the string \\\\servername\\dir1\\subdir1\\, you use:
string dir = #"\\\\servername\\dir1\\subdir1\\";
or:
string dir = "\\\\\\\\servername\\\\dir1\\\\subdir1\\\\";

This string "\\\\servername\\dir1\\subdir1\\" is the same as #"\\servername\dir1\subdir1\". In order to escape backslashes you need either use # symbol before string, or use double backslash instead of one.
Why you need that? Because in C# backslash used for escape sequences.

How to quote \" (slash double-quote) in a string literal?

This is probably a really simple question but I can't seem to get my head around it. I need to have a string that contains \" without it seeing it as an escape character. I tried using # but it won't work. The only other way I thought of doing this would be to use \u0022 but don't want to unless I can help it.
Desired string - string s = "\"\""; // Obviously this doesn't work!
Desired console output - \"\"
Hope this makes sense.
Thanks!

Try
string s = "\\\"\\\"";
You have to escape your backslashes too.
Mike

You can use the literal, but you need to double-up quotes.
string s = #"\""\""";

In verbatim string literals (#"...") a " in the string value is encoded as "", which happens to also be the only escape sequence in verbatim strings.
#"\""Happy coding!\""" // => \"Happy coding!\"
"\\\"Happy coding!\\\"" // => \"Happy coding!\"
Note that in the 2nd case (not a verbatim string literal), a \ is required before the \ and the " to escape them and prevent their normal meanings.
See the C# string reference for more details and examples.

I think you have to escape backslashes too... so something like "\\\"\\\"" should work, I believe.

Use this string:
string s = "\\\"\\\"";

Console.WriteLine( "\\\"\\\"" );
Just put a \ before each character that needs to be printed.

String s = #"\""\""";
DblQuote characters will escape a second dblquote character
Though for better readability I would go with:
const String DOUBLEQUOTE = """";
const String BACKSLASH = #"\";
String s = BACKSLASH + DOUBLEQUITE + BACKSLASH + DOUBLEQUOTE;

In a verbatim string (a string starting with #"") to escape double quotes you use double quotes, e.g. #"Please press ""Ok"".". If you want to do it with verbatim strings then you would do something like #"\""" (that's 3 double quotes on the end there).

You can do like this,
string s = "something'\\\'";
Use a single '' rather then "" in string to do the same.

Problem with String Verbatim

I use C#.
I need assign a value to a string as verbatim.
Here my code:
string verbatim = "#<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">";
or
string verbatim = #"<META NAME=""ROBOTS"" CONTENT=""NOINDEX, NOFOLLOW"">";
But it does not work.
What I'm doing wrong here? Thanks

You mean a verbatim string literal? Double-up the internal quotes and move the #:
string verbatim = #"<META NAME=""ROBOTS"" CONTENT=""NOINDEX, NOFOLLOW"">";

The # charctaer goes outside the string ath beginning and you need to escape your quotes, i.e.
string verbatim = #"<META NAME=""ROBOTS"" CONTENT=""NOINDEX, NOFOLLOW"">"

The # must be outside the string and you need to use double quotes:
string verbatim = #"<META NAME=""ROBOTS"" CONTENT=""NOINDEX, NOFOLLOW"">";

string replace single quote to double quote in C#

How can I replace a single quote (') with a double quote (") in a string in C#?

You need to use the correct escape sequence for the quotes symbol, you can find more information about escape sequencies here.
String stringWithSingleQuotes= "src='http://...';";
String withDoubleQuotes = stringWithSingleQuotes.Replace("'","\"");

var abc = "hello the're";
abc = abc.Replace("'","\"");

string str;
str=strtext.Replace('"','\'');