Regex that accepts only numbers (0-9) and NO characters [duplicate]

I need a regex that will accept only digits from 0-9 and nothing else. No letters, no characters.

I thought this would work:

^[0-9]

or even

\d+

but these are accepting the characters : ^,$,(,), etc

I thought that both the regexes above would do the trick and I’m not sure why its accepting those characters.

EDIT:

This is exactly what I am doing:

 private void OnTextChanged(object sender, EventArgs e)
    {

   if (!System.Text.RegularExpressions.Regex.IsMatch("^[0-9]", textbox.Text))
        {
            textbox.Text = string.Empty;
        }
    }

This is allowing the characters I mentioned above.