Is my new command incompatible with hyperref?

Question

I have this command that works fine:

\newcommand{\mytag}[2]{%
    \text{#1}%
    \@bsphack
    \protected@write\@auxout{}%
    {\string\newlabel{#2}{{#1}{\thepage}}}%
    \@esphack   
}

but when I add the package hyperref as:

\usepackage{hyperref}
\hypersetup{
    colorlinks=true, %set true if you want colored links
    linktoc=page,     %set to all if you want both sections and subsections linked
    linkcolor=blue,  %choose some color if you want links to stand out
}

Then latex prompts me this error when I'm using my newcommand and does not compile. The error is:

Paragraph ended before \Hy@setref@link was complete.

I tried to remove all the .aux files but the error does not dissapear. It only dissapears when I comment the hyperref lines. How can I solve this problem?

EDIT:

\documentclass{article}
\usepackage [english] {babel}
\usepackage {anysize}
\usepackage [T1]{fontenc}
\usepackage {amsmath, amsthm, amsfonts, amssymb}
\usepackage [utf8]{inputenc}
\usepackage{mathtools}

\usepackage{lipsum}
\usepackage{hyperref} %If you comment this, it works. If you uncomment it doesn't

\makeatletter

%my command
\newcommand{\mytag}[2]{%
    \text{#1}%
    \@bsphack
    \protected@write\@auxout{}%
    {\string\newlabel{#2}{{#1}{\thepage}}}%
    \@esphack   
}

%frougon command. It does not work as expected. It loses the posibility to refer to portions of the equations.
%\newcommand*{\mytag}[2]{\text{#1}\def\@currentlabel{#1}\label{#2}}
\makeatother

\begin{document}
\begin{equation}\label{eq:myeq}
    E=\underbrace{\underbrace{mv^2}_{\mytag{kinetic}{eq:myeq_kinetic}}+\underbrace{mgh}_{\mytag{potential}{eq:myeq_potential}}}_{\mytag{total Energy}{eq:myeq_totalE}}, 
\end{equation}

Eq. \ref{eq:myeq} is the expresion for \ref{eq:myeq_totalE}, which is composed by \ref{eq:myeq_kinetic} and \ref{eq:myeq_potential} energies.

\end{document}

Answer 1

hyperref redefines a few LaTeX commands, including \label. Indeed, it needs to store more data associated to each label in the .aux file, in order to provide the features it offers.

Your command is essentially \text{#1} followed by the definition of the \label command from the LaTeX kernel. Thus, it writes data to the .aux file in a format that is incompatible with hyperref. In order to avoid such compatibility problems, the easiest way is to simply use \label to do the delicate work (the one that has to be different according to whether hyperref is loaded or not). Since the definition of \label in the LaTeX kernel is:

\def\label#1{\@bsphack
  \protected@write\@auxout{}%
         {\string\newlabel{#1}{{\@currentlabel}{\thepage}}}%
  \@esphack}

and since amsmath also renames \label to \ltx@label, your command can be translated as follows:

\documentclass{article}
\usepackage{amsmath}
\usepackage{hyperref}

\makeatletter
\newcommand*{\mytag}[2]{\text{#1}\def\@currentlabel{#1}\ltx@label{#2}}
\makeatother

\begin{document}

\begin{equation}\label{eq:myeq}
    E = \underbrace{
          \underbrace{\frac{1}{2} mv^2}_{\mytag{kinetic}{eq:myeq_kinetic}}
        + \underbrace{mgh}_{\mytag{potential}{eq:myeq_potential}}
        }_{\mytag{mechanical energy}{eq:myeq_mechanicalE}}
\end{equation}

Eq.~\ref{eq:myeq} is the expression for~\ref{eq:myeq_mechanicalE}, which is
composed of~\ref{eq:myeq_kinetic} and~\ref{eq:myeq_potential} energies.

\end{document}

Note: as @DavidCarlisle pointed out, your \@bsphack ... \@esphack pair was useless in math mode.