\allowbreak does not work in \index

Question

I would like to include \allowbreak in an \index entry but I can't get it to work. The \allowbreak is changed to \penalty \z@ in the .idx file, which corrupts the generated index.

How can I solve this?

Answer 1

You are probably calling \index in the argument to another command; here's an emulation:

\documentclass{article}
\usepackage{makeidx}
\makeindex

\begin{document}

a\index{abcdef@abc\allowbreak def}

% now do the call in the argument of a command
\textbf{g\index{ghijkl@ghi\allowbreak jkl}}

% now use the proper input
\textbf{m\index{mnopqr@mno\protect\allowbreak pqr}}

\printindex

\end{document}

This produces the following .idx file

\indexentry{abcdef@abc\allowbreak def}{1}
\indexentry{ghijkl@ghi\penalty \z@ jkl}{1}
\indexentry{mnopqr@mno\allowbreak pqr}{1}

The same problem with \allowbreak turned into \penalty \z@ would happen if you have something like

\newcommand{\indexword}[1]{#1\index{#1}}

and do

\indexword{abc\allowbreak def}

Using \protect\allowbreak will do also in this case. However, sorting of index entries with \allowbreak in them will probably give wrong results.