Getting square brackets inside bibtex year field

Question

I'm trying to get square brackets to appear inside the year field in bibtex, but they disappear in the output. Here's what I've tried:

year = {2000 [1939]}
year = {2000 \[1939\]}  
year = {2000 {[}1939{]}}

The expected output is:

• Smith (2000 [1939])

But instead I get:

• Smith (2000 1939)

I'm using the Association for Computational Linguistics (ACL) style (\bibliographystyle{acl}), which clashes with natbib. A copy can be found here for example:

https://github.com/jonsafari/acl.bst/blob/master/acl.bst

I'm not sure how to provide a MWE - I could paste the whole acl.bst here, but it's 1300 lines... Sorry, new to tex exchange!

Here is an entire package with a sample document to reproduce this in:

http://coling2018.org/wp-content/uploads/2018/01/coling2018.zip

An entry like this produces the problem:

@Book{Smith2000,
  title                    = {Some Book},
  author                   = {W. E. Smith},
  publisher                = {Clarendon Press},
  year                     = {2000 [1939]},
  address                  = {Oxford}
}

Answer 1

Consider the following MWE

\documentclass[11pt]{article}
\usepackage{coling2018}
\usepackage{times}
\usepackage{url}

\begin{filecontents}{\jobname.bib}
@Book{Smith2000,
  title      = {Some Book},
  author     = {W. E. Smith},
  publisher  = {Clarendon Press}, 
  year       = {2000 [1939]},
  address    = {Oxford}
}
\end{filecontents}

\begin{document}
\cite{Smith2000}
\bibliographystyle{acl}
\bibliography{\jobname}
\end{document}

which produces the .bbl entry

\bibitem[\protect\citename{Smith}2000 1939]{Smith2000}
W.~E. Smith.
\newblock 2000 [1939].
\newblock {\em Some Book}.
\newblock Clarendon Press, Oxford.

and the output

The culprit is ll. 1100-1103 of acl.bst

  "\protect\citename{" swap$ * "}" *
  year field.or.null purify$ *
  'label :=
  year field.or.null purify$ *

In particular the behaviour you see is caused by purify$. That function (page 7 of Designing BibTeX Styles)

removes nonalphanumeric characters except for white-space characters and hyphens and ties (these all get converted to a space) [and] removes certain alphabetic characters contained in the control sequences associated with a “special character”

So because your year field gets purify$'d it loses its brackets. That means we need to stop acl.bst purifying your year. That is, however, not enough because with the raw year you would end up with

\bibitem[\protect\citename{Smith}2000 [1939]]{Smith2000}
W.~E. Smith.
\newblock 2000 [1939].
\newblock {\em Some Book}.
\newblock Clarendon Press, Oxford.

in the .bbl, that will lead to errors because we can't have unprotected square brackets in the optional argument, see ] inside an optional argument. So we need to brace the entire year field to be sure.

You will have to modify the .bst style to get the output you want. For paper submissions that might not be the best idea, because you probably can't send your modified .bst file, but it will work if the .bbl is sent along or just pasted into the document.

1. Copy acl.bst to a location where TeX can find it. The document directory will do fine.
2. Rename the file to acl-rawyear.bst

3. Open the file and find the function FUNCTION {calc.label}, replace its second block (ll. 1100-1103)

     "\protect\citename{" swap$ * "}" *
     year field.or.null purify$ *
     'label :=
     year field.or.null purify$ *
   

   with

     "\protect\citename{" swap$ * "}" *
     "{" * year field.or.null * "}" *
     'label :=
     label *
   

4. Add your name and date and explain the modification in the comments at the top of the file.

5. Use \bibliographystyle{acl-rawyear} instead of \bibliographystyle{acl} in your document.

Now

%\RequirePackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Book{Smith2000,
  title      = {Some Book},
  author     = {W. E. Smith},
  publisher  = {Clarendon Press}, 
  address    = {Oxford},
  year       = {2000 [1939]},
}
@Book{elk:bronto,
  title      = {Bronto},
  author     = {Anne Elk},
  publisher  = {Clarendon Press}, 
  address    = {Oxford},
  year       = {1972},
}
@Book{elk:trice,
  title      = {Triceratops},
  author     = {Anne Elk},
  publisher  = {Clarendon Press}, 
  address    = {Oxford},
  year       = {1972},
}
@Book{elk:ptero,
  title      = {Pterodactylus},
  author     = {Anne Elk},
  publisher  = {Clarendon Press}, 
  address    = {Oxford},
  year       = {1973},
}
\end{filecontents}

\documentclass{article}
\usepackage{coling2018}

\begin{document}
\cite{Smith2000,elk:bronto,elk:trice,elk:ptero}
\bibliographystyle{acl-rawyear}
\bibliography{\jobname}
\end{document}

outputs