A `\#` in the first argument of `\href` produces `/#` instead of `#`

Created by: pdmosses

Issue Summary

To define a macro \mylink with target id as parameter, such that \mylink{a} expands to \href{#a}{a}, the use of # in the macro definition needs to be escaped, to avoid ambiguity with LaTeX parameter references such as #1 (and potentially ##1 for nested macros).

Using \# works in a macro definition in LaTeX (when using the hyperref package):

\newcommand{\mylink}[1]{\href{\##1}{#1}}

but the same definition does not give the expected result in MathJax.

Steps to Reproduce:

This is the first step

$$
\newcommand{\mylink}[1]{\href{\##1}{#1}}
$$

Target math `\cssId{a}{A}`
$$
\cssId{a}{A}
$$

`\href{#a}{b}` gives a correct hyperlink to `#a`:

$$
\href{#a}{b}
$$

`\mylink{a}` gives an incorrect link to `localhost:4000/#a`:

$$
\mylink{a}
$$

This is the second step

Check that the link to the target from b is correct, and the link from a is broken.

I would expect MathJax to follow LaTeX by allowing the use of \# for # in the URL argument of \href in a macro definition. MathJax accepts the use of \#, but does not interpret it as # in the URL argument of \href.

Technical details:

MathJax Version: 3.1.2
Client OS: macOS 10.15.7
Browser: Safari 14.0
Framework: Jekyll 4.1.1, using MathJax inside kramdown math blocks $$...$$

Supporting information:

Include the MathJax configuration you are using, and the script tag that loads MathJax itself.

  <script>
    MathJax = { 
      tex: { 
        tags: 'ams',
        packages: {'[+]': ['textmacros']}
      },
      svg: { 
        fontCache: 'global', 
        displayAlign: 'left', 
        displayIndent: '2em',
        mtextInheritFont: true
      },
      loader: {
        load: ['[tex]/textmacros']
      }
    };
  </script>
  <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
  <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg.js"></script>

I am a new user of MathJax. I am experimenting with using MathJax in the Jekyll theme Just the Docs.

P.S. Here is the HTML generated by Jekyll/kramdown from the example in step 1 above:

\[\newcommand{\mylink}[1]{\href{\##1}{#1}}\]
<p>Target math <code class="language-plaintext highlighter-rouge">\cssId{a}{A}</code>
\(\cssId{a}{A}\)</p>
<p><code class="language-plaintext highlighter-rouge">\href{#a}{b}</code> gives a correct hyperlink to <code class="language-plaintext highlighter-rouge">#a</code>:</p>
\[\href{#a}{b}\]
<p><code class="language-plaintext highlighter-rouge">\mylink{a}</code> gives an incorrect link to <code class="language-plaintext highlighter-rouge">localhost:4000/#a</code>:</p>
\[\mylink{a}\]