making soulutf8 work with \(...\)

Question

Is there any easy way to adapt soulutf8 to work with LaTeX math mode delimiters?

As of now, compiling

\documentclass{article}
\usepackage{xcolor,soulutf8}
\begin{document}
Well, \hl{\(e^{i\pi}=-1\)}.
\end{document}

with pdflatex results in

! Extra }, or forgotten $.
\SOUL@doword ...hskip \z@ \relax \the \SOUL@word }
                                                  \let \SOUL@errmsg \SOUL@er...
l.4 Well, \hl{\(e^{i\pi}=-1\)}
                              .
? X

Let us not go into the discussion on which one is better: $...$ or \(...\); take it for granted that the typesetter has his/her reasons to prefer the second pair of delimiters.

P.S. As Heiko pointed out, the same problem exists in soul (which unfortunately seems to lack an active maintainer as of now), whose documentation specifically mentions this limitation.

Answer 1

One could parse the argument of \hl to replace all \(...\) by $...$ in that argument and then use the original definition of \hl. I'm not entirely sure whether the following is completely stable, though it worked for everything I've thrown at it so far.

Edit: minimise code by removing case which is never matched (##3 is dot and ##2 is not) and not include the \(.\) tokens in every recursion of the replacement but only once (which suffices as those are in ##3 in the case of a recursion).

Note that there are cases of valid but strange syntax (if used outside of the argument of \hl) which fail here: If one mixes $ and one of \( or \) as the delimiters the provided code fails; though $a^b\) or \(a^b$ would be valid syntax \hl{$a^b\)} and \hl{\(a^b$} would fail as they don't match the required argument pattern of \hl@grab.

\documentclass{article}
\usepackage{xcolor,soulutf8}
\usepackage[]{amsmath}
\usepackage{letltxmacro}
\LetLtxMacro\hlBAK\hl
\begingroup
\makeatletter
\catcode`.=4% changing catcode so the dot is never matched by ordinary text
\def\zz%
  {\endgroup
    \renewcommand\hl[1]
      {%
        \hl@grab##1\(.\)\endhl@grab
      }%
    \long\def\hl@grab##1\(##2\)##3\endhl@grab%
      {%
        \hl@grab@ifdot{##2}
          {\hlBAK{##1}}
          {\hl@grab##1$##2$##3\endhl@grab}%
      }%
    \long\def\hl@grab@ifdot##1%
      {%
        \ifx.##1%
          \expandafter\@firstoftwo
        \else
          \expandafter\@secondoftwo
        \fi
      }%
  }%
\zz
\begin{document}
Well, \hl{\(.e^{i\pi}=-1\) text \(2=2\).}
\end{document}

EDIT: The following provides a replacement using two macros, one to replace \( and one to replace \). As a result the strange (but technically correct) syntax of \(a^b$ and $a^b\) should also work as an argument to \hl (though this way \hl{\(a^b\( text $a^b$} would work, too, which is obviously not correct syntax).

\documentclass{article}
\usepackage{xcolor,soulutf8}
\usepackage[]{amsmath}
\usepackage{letltxmacro}
\LetLtxMacro\hlBAK\hl
\begingroup
\makeatletter
\catcode`.=4% changing catcode so the dot is never matched by ordinary text
\def\zz%
  {\endgroup
    \renewcommand\hl[1]
      {%
        \hl@grab@A##1\(.\endhl@grab
      }%
    \long\def\hl@grab##1\(##2\)##3\endhl@grab%
      {%
        \hl@grab@ifdot{##2}
          {\hlBAK{##1}}
          {\hl@grab##1$##2$##3\endhl@grab}%
      }%
    \long\def\hl@grab@A##1\(##2\endhl@grab%\)
      {%
        \hl@grab@ifdot{##2}
          {\hl@grab@B##1\).\endhl@grab}
          {\hl@grab@A##1$##2\endhl@grab}%
      }%
    \long\def\hl@grab@B##1\)##2\endhl@grab%
      {%
        \hl@grab@ifdot{##2}
          {\hlBAK{##1}}
          {\hl@grab@B##1$##2\endhl@grab}%
      }%
    \long\def\hl@grab@ifdot##1%
      {%
        \ifx.##1%
          \expandafter\@firstoftwo
        \else
          \expandafter\@secondoftwo
        \fi
      }%
  }%
\zz
\begin{document}
Well, \hl{\(.e^{i\pi}=-1\) text \(2=2\).}
\hl{$a^b)}\hl{(a^b$}
\hl{(a (b$ $}
\end{document}

---

Using etl as a faster, yet less versatile alternative to l3regex to also replace \( and \) inside of braces:

\documentclass{article}
\usepackage{xcolor,soul}
\usepackage{etl}
\NewCommandCopy\hlBAK\hl
\ExplSyntaxOn
\renewcommand\hl[1]
  {
    \exp_args:Ne \hlBAK
      {
        \etl_token_replace_all_deep:eNn
          { \etl_token_replace_all_deep:nNn {#1} \( { $ } }
          \) { $ }
      }
  }
\cs_generate_variant:Nn \etl_token_replace_all_deep:nNn { e }
\ExplSyntaxOff
\begin{document}
\hl{\emph{Some important ( m = ath )}}
\end{document}

Answer 2

With the l3regex module it's quite easy and works for all soul commands. The idea is to redefine \SOUL@start, which is the macro that does all the business according to settings already made by the called macros.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor,soulutf8}
\ExplSyntaxOn
\tl_new:N __l_SOUL_argument_tl
\cs_set_eq:Nc \SOUL_start:n { SOUL@start }
\cs_generate_variant:Nn \SOUL_start:n { V }
\cs_set_protected:cpn {SOUL@start} #1
 {
  \tl_set:Nn __l_SOUL_argument_tl { #1 }
  \regex_replace_all:nnN
   { \c{(} (.*?) \c{)} } % look for (...) (lazily)
   { \cM$ \1 \cM$ }      % replace with $...$
   __l_SOUL_argument_tl
  \SOUL_start:V __l_SOUL_argument_tl % do the usual
 }
\ExplSyntaxOff
\begin{document}
Well, \hl{(e^{i\pi}=-1) text (2=2).}
Well, \ul{(e^{i\pi}=-1) text (2=2).}
Well, \so{(e^{i\pi}=-1) text (2=2).}
Well, \caps{(e^{i\pi}=-1) text (2=2).}
Well, \st{(e^{i\pi}=-1) text (2=2).}
\end{document}

Answer 3

The use of \( and \) is not supported by package soul, from the documentation:

§ 3 Mathematics:

Example: \so{foo$x^3$bar}

Mathematic formulas are allowed, as long as they are surrounded by $.

Note that the LaTeX equivalent \(...\) does not work.

Thus, the workaround with $ remains:

\hl{$e^{i\pi}=-1$}