Why can't I use \edef with \pageref from hyperref?

Question

This works nicely:

\documentclass{article}
\begin{document}
    \label{mylabel}
    \pageref{mylabel}
    \edef\mypage{\pageref{mylabel}}
\end{document}

But using hyperref, thus replacing \pageref by \pageref* to have only the page number without the link, this does not work:

\documentclass{article}
\usepackage{hyperref}
\begin{document}
    \label{mylabel}
    \pageref*{mylabel}
    \edef\mypage{\pageref*{mylabel}}
\end{document}

The error is:

! Undefined control sequence.
l.6     \edef\mypage
                 {\pageref*{mylabel}}
The control sequence at the end of the top line
of your error message was never \def'ed.

Why is that, and how can I do it better?

Answer 1

Commands taking a *-variant are not expandable, so they can't be used in \edef.

The particular error is not really meaningful; if you go past it and try running LaTeX again, you get

! Undefined control sequence.
\HyRef@StarSetRef ...Hy@safe@activestrue \edef \x 

You get what you'd like with refcount and \getpagerefnumber:

\documentclass{article}
\usepackage{refcount}
\usepackage{hyperref}

\begin{document}

\label{mylabel}
\pageref*{mylabel}
\edef\mypage{\getpagerefnumber{mylabel}}

\mypage

\end{document}

The output shows 1 twice as expected.

Using \protected@edef as suggested in another answer is not what you're looking for. Here's an extended example:

\documentclass{article}
\usepackage{refcount}
\usepackage{hyperref}

\begin{document}

\label{mylabel}
\pageref*{mylabel}
\edef\mypage{\getpagerefnumber{mylabel}}

\mypage

\makeatletter
\protected@edef\mypageagain{\pageref*{mylabel}}
\makeatother

\texttt{\meaning\mypage}

\texttt{\meaning\mypageagain}

\end{document}

You can see that \mypageagain doesn't contain the page number.

Answer 2

You can not, in general, use \edef on general latex text, the fact that you do not get an error in the first use is purely by chance. More or less any * command is going to fail as \@ifnextchar does not work by expansion so it will fail in the lookahead for the * irrespective of the command definition.

This is the origin of the "fragile command" distinction in LaTeX and why latex commands should use \protected@edef so \protect is set up to make fragile commands safe before the edef. (But being safe means it may not expand as you need, you did not give your actual use case here)