Question

I am having trouble creating a regular expression to disallow the following four characters and limit the size:

/
#
?
\

What I currently have is:

Regex regex = new Regex("^[^/\\#?]{0,1024}$", RegexOptions.Compiled);
if (!regex.IsMatch("\\"))
{
    Console.WriteLine("Bad");
}

All of the characters except \ are disallowed. I cannot get \ to work.

Any suggestions on how to support this?

Answer 1

Your regex is fine, ^[^/\\#?]{0,1024}$.

However, in C# backslash is an escape character, so a C# "\\" is a single backslash.
Hence for each backslash in your regex, you have to backslash again for C#:

Regex regex = new Regex("^[^/\\\\#?]{0,1024}$", RegexOptions.Compiled);

Alternatively, you can use a raw string, meaning backslashes in C# strings remain backslashes (note the @ symbol):

Regex regex = new Regex(@"^[^/\\#?]{0,1024}$", RegexOptions.Compiled);