Regex to remove all special characters from string?

It really depends on your definition of special characters. I find that a whitelist rather than a blacklist is the best approach in most situations:

tmp = Regex.Replace(n, "[^0-9a-zA-Z]+", "");

You should be careful with your current approach because the following two items will be converted to the same string and will therefore be indistinguishable:

"TRA-12:123"
"TRA-121:23"

This should do it:

[^a-zA-Z0-9]

Basically it matches all non-alphanumeric characters.

[^a-zA-Z0-9] is a character class matches any non-alphanumeric characters.

Alternatively, [^\w\d] does the same thing.

Usage:

string regExp = "[^\w\d]";
string tmp = Regex.Replace(n, regExp, "");

You can use:

string regExp = "\\W";

This is equivalent to Daniel's "[^a-zA-Z0-9]"

\W matches any nonword character. Equivalent to the Unicode categories [^\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Nd}\p{Pc}].

For my purposes I wanted all English ASCII chars, so this worked.

html = Regex.Replace(html, "[^\x00-\x80]+", "")