[tex4ht] ! Bad register code (65536)., \__regex_query_set:nnn ... \l__regex_curr_pos_int MATHJAX mode

[Nasser M. Abbasi]

Hello;

I have a MWE with one long equation generated by computer algebra system.

----------------------------------
\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}
\DeclareMathOperator{\EllipticF}{EllipticF}
\DeclareMathOperator{\EllipticE}{EllipticE}
\begin{document}
\[
... long equation here
\]
\end{document}
----------------------

The file compiles OK using lualatex foo.tex.

When compiled using

make4ht --lua -u foo.tex "htm,0,notoc*,pic-align"

It also generates HTML file OK, but with warning that
seems harmless
---------------------
[1 file foo.htm
  file foo.css
  file foo.tmp
--- warning --- Too many characters (> 500) for map line: `2'
]
---------------------

but here is the real problem: When I compile in mathjax mode, it
gives (my actual command uses my own .cfg, but it is not needed
to show this error)

make4ht -ulm default foo.tex
    "htm,0,mathjax,notoc*,p-width,charset=utf-8" " -cunihtf -uth8"

----------------------------------
(/usr/local/texlive/2018/texmf-dist/tex/generic/tex4ht/html4.4ht)
(/usr/local/texlive/2018/texmf-dist/tex/generic/tex4ht/unicode.4ht)
(./html4-math.4ht)
(/usr/local/texlive/2018/texmf-dist/tex/generic/tex4ht/html5.4ht)) (./foo.aux)
! Bad register code (65536).
\__regex_query_set:nnn ... \l__regex_curr_pos_int
                                                   {#1}\int_incr:N \l__regex_...

l.9 \]
-----------------------------------

my texmf.cnf is

----------------------------
>cat /usr/local/texlive/2018/texmf.cnf

buf_size=90000000
pool_size=9000000
main_memory=8000000
-------------------------

I know the problem is due to large size of the equation.
Editor tells me it is 208,987 characters long so I have
to remove it.   But the question is why this error in mathjax
only?

Please find the file foo.tex  and the log file in
this folder on my site

https://www.12000.org/tmp/03122019/

I am using TL 2018, compiling on ubuntu under windows 10.

>which make4ht
/usr/local/texlive/2018/bin/x86_64-linux/make4ht
>make4ht -v
make4ht version v0.2c

Thanks
--Nasser