Removing characters from a string

[WirelessDice]

Okay, what I basically want to do is to remove the two last characters from my string witch is a
tab and a newline character, but I think that doesn't matter, because they are always at the end of
the string. I got a code already, but it doesn't work correctly.

    string::size_type pos = 0;
    while ((pos = txt.find_first_of("\n\t", pos) != string::npos))
        txt.erase(pos, 1);

    return txt;

It seems like that code removes everything except the first character.
Thanks in advance