LuaLaTeX and \big, \bigg, \Big, \Bigg: Change the math class of the symbol (i.e. change spacing)

Question

I am referring to David Carlisle's answer to the question on how one can achieve the same spacing around \middle| as around \mid. Please note, that I am using unicode-math and LuaLaTeX.

The answer is to use the unicode versions \Uleft, \Umiddle and \Uright and state the class explicitly, e.g.

\documentclass{article}
\begin{document}
$\Uleft( A \Umiddle class 2 | B \Uright)$
\end{document}

Now, I have

\renewcommand{\left}{\Uleft}
\renewcommand{\right}{\Uright}
\renewcommand{\middle}{\Umiddle class 2 }

after \begin{document} and I am quite happy with the spacing.

However, sometimes I need to resort to the explicit sizes \big, \bigg, ... and so on and their respective left and right counterparts. My naive hope was that their are also unicode variants and \Ubig class 2 \vert would do the trick. But those commands do not exist.

How do I get a class-2 spacing for \big, \bigg, ... and so on?

Answer 1

since you want a class 2 (\mathbin) \Big | you can simply use

 a  \mathbin{\Big|}  b

which would work in luatex as in pdftex.

for class3 (\mathrel) latex already defines \Bigm.