Natbib \citet style: move optional argument outside bracket

Question

I am trying to use bibtex+natbib to deal with the citations in my thesis. Some citations are of the style \citet{author_year} and work as expected.

However, when I use \citet[pXX]{author_year}, then I get Author (Year, pXX) instead of the desired Author (Year), pXX

How can I achieve the desired style for this second kind of citations with additional information like page number etc.

Based on the first answer from @Audrey I made this MWE:

%this documents is compiled with xetex
\documentclass[11pt,british,english,ngerman,footnotes=multiple]{scrreprt}
\usepackage{fontspec}
\usepackage[a4paper]{geometry}
\geometry{verbose,tmargin=2cm,bmargin=2cm,lmargin=3cm,rmargin=3cm}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\usepackage{array}
\usepackage{rotfloat}
\usepackage{booktabs}
\usepackage{footmisc}
\usepackage{graphicx}
\usepackage{setspace}
\usepackage[authoryear]{natbib}
\PassOptionsToPackage{normalem}{ulem}
\usepackage{ulem}
\onehalfspacing

\usepackage{babel}
%\usepackage{hyperref}
\usepackage{apacite}
\usepackage{makerobust}
\DeclareRobustCommand{\citep}[2][]{\cite[#1]{#2}}
\DeclareRobustCommand{\citealp}[2][]{\citeNP[#1]{#2}}
\DeclareRobustCommand{\citet}[2][]{\citeA[#1]{#2}}

\usepackage{chngcntr}
\counterwithout{figure}{chapter}
\counterwithout{table}{chapter}
\counterwithout{footnote}{chapter}

\bibpunct[, ]{(}{)}{,}{a}{,}{,}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\NAT@citex}
  {\ifNAT@swa\else\if*#2*\else\NAT@cmt#2\fi
   \if\relax\NAT@date\relax\else\NAT@@close\fi\fi}
  {\ifNAT@swa\else\if*#2*\else\ifNAT@par\else\NAT@cmt#2\fi\fi
   \if\relax\NAT@date\relax\else\NAT@@close%
   \if*#2*\else\ifNAT@par\NAT@cmt#2\fi\fi\fi\fi}{}{}
\makeatother

\begin{filecontents}{\jobname.bib}
@Book{companion,
  author = {Goossens, Michel and Mittelbach, Frank and Samarin, Alexander},
  title = {The LaTeX Companion},
  edition = {1},
  publisher = {Addison-Wesley},
  location = {Reading, Mass.},
  year = {1994}}
@Book{adams,
  title = {The Restaurant at the End of the Universe},
  author = {Douglas Adams},
  series = {The Hitchhiker's Guide to the Galaxy},
  publisher = {Pan Macmillan},
  year = {1980}}
\end{filecontents}

\begin{document}
\noindent
\cmd{citet}: \citet[e.g.][100]{adams}, \citet[pp. 1--10]{companion,adams} \\
\cmd{Citet}: \Citet[pp. 1--10]{adams}, \Citet[pp. 1--10]{companion} \\
\cmd{citet*}: \citet*[pp. 1--10]{companion,adams} \\
\cmd{Citet*}: \Citet*[p. 10]{companion} \\
\cmd{cite}: \cite[e.g.][100]{adams}, \cite{companion} \\
\cmd{citep}: \citep[e.g.][p. 20]{adams}, \citep[1--10]{companion,adams} \\
\cmd{citetext}, \cmd{citealp}:
\citetext{see \citealp{companion}, or even better \citealp{adams}} \\
\cmd{citeauthor}: \citeauthor{adams}, \citeauthor{companion} \\
\cmd{citeyear}: \citeyear{adams}, \citeyear{companion} \\
\bibliographystyle{plainnat}
\bibliography{\jobname}
\end{document}

Does anyone understand how to tame the apacite beast in combination with natbib?

Answer 1

With etoolbox you can patch the command natbib uses to set all non-numeric citation labels: \NAT@citex. The postnote is passed to \NAT@citex as the second argument (#2). For \citet and its variants \ifNAT@swa will expand <false>, \ifNAT@par expands <true> and \NAT@date is defined.

\documentclass{article}
\usepackage{natbib}
\usepackage{etoolbox}

\makeatletter
\patchcmd{\NAT@citex}
  {\ifNAT@swa\else\if*#2*\else\NAT@cmt#2\fi
   \if\relax\NAT@date\relax\else\NAT@@close\fi\fi}
  {\ifNAT@swa\else\if*#2*\else\ifNAT@par\else\NAT@cmt#2\fi\fi
   \if\relax\NAT@date\relax\else\NAT@@close%
   \if*#2*\else\ifNAT@par\NAT@cmt#2\fi\fi\fi\fi}{}{}
\makeatother

\begin{filecontents}{\jobname.bib}
@Book{companion,
  author = {Goossens, Michel and Mittelbach, Frank and Samarin, Alexander},
  title = {The LaTeX Companion},
  edition = {1},
  publisher = {Addison-Wesley},
  location = {Reading, Mass.},
  year = {1994}}
@Book{adams,
  title = {The Restaurant at the End of the Universe},
  author = {Douglas Adams},
  series = {The Hitchhiker's Guide to the Galaxy},
  publisher = {Pan Macmillan},
  year = {1980}}
\end{filecontents}

\newcommand{\cmd}[1]{\textbackslash\texttt{#1}}
\begin{document}
\noindent
\cmd{citet}: \citet[e.g.][100]{adams}, \citet[pp. 1--10]{companion,adams} \\
\cmd{Citet}: \Citet[pp. 1--10]{adams}, \Citet[pp. 1--10]{companion} \\
\cmd{citet*}: \citet*[pp. 1--10]{companion,adams} \\
\cmd{Citet*}: \Citet*[p. 10]{companion} \\
\cmd{cite}: \cite[e.g.][100]{adams}, \cite{companion} \\
\cmd{citep}: \citep[e.g.][p. 20]{adams}, \citep[1--10]{companion,adams} \\
\cmd{citetext}, \cmd{citealp}:
\citetext{see \citealp{companion}, or even better \citealp{adams}} \\
\cmd{citeauthor}: \citeauthor{adams}, \citeauthor{companion} \\
\cmd{citeyear}: \citeyear{adams}, \citeyear{companion} \\
\bibliographystyle{plainnat}
\bibliography{\jobname}
\end{document}