How to remove parenthesis in an equation number created by \tag?

Question

This is probably simple, but I can't figure it out. How can I remove the parentheses around a \tag in an align equation? I would like the second line (and only that) to contain [2] instead of ([2]). A workaround would be to give it no label, to see the label [2] as part of the equation, and move it to the right text margin by hand. But how? \hfill does not insert space here.

\documentclass{scrartcl}

\usepackage[T1]{fontenc}
\usepackage[american,ngerman]{babel}
\usepackage{amsmath}

\begin{document}
\begin{align}
  a &= b \\
  c &= d \tag{[2]}
\end{align}
\end{document}

Answer 1

As egreg said, use the starred version \tag* which is the same as \tag except that it does not automatically enclose the tag in parentheses.

\documentclass{article}

\usepackage{amsmath}

\begin{document}
    \begin{equation}
        a + b = c \tag*{[2]}
    \end{equation}
\end{document}