\middle preceded by primed symbol gives an error.

[nberth]

The following code gives me an error "\middle must be within \left and \right" when I render it in my browser (Firefox 21.0, MathJax v2.4.0):

• \left\{q\in Q \middle| P(q)\right\} works;
• \left\{q\in Q' \middle| P(q)\right\} doesn't, nor \left\{q\in Q'' \middle| P(q)\right\} incidentally.
  This also happens when Q' is the result of macro expansion:
• \newcommand{\Q}{Q} \left\{q\in \Q \middle| P(q)\right\} works;
• \newcommand{\Q}{Q'} \left\{q\in \Q \middle| P(q)\right\} doesn't.

The error is reproducible (though no error message is displayed) on the dynamic rendering page [1], yet I did not try it on the development branch for lack of time to install it (can anyone try?).

[1] http://cdn.mathjax.org/mathjax/latest/test/sample-dynamic.html

[pkra]

Thank you for reporting the issue! (I've modified your posting to escape the TeX). We'll look into it.

[dpvc]

For now you can use

\left\{q\in {Q'} \middle| P(q)\right\}

to get the result you are looking for. There is a problem with how \middle detects whether it is within \left...\right (obviously).

[dpvc]

The issue1023 branch of my fork of MathJax resolves this issue (as well as a potential spacing problem with \middle).

[dpvc]

==> Merged.

[dpvc]

==> In testsuite

LaTeXToMathML/delimiters/issue1023.html