Concatenating regex variables in fieldvalue

Question

Friends, I'm stuck with a doubt about how to concatenate values from regex parenthesis into the fieldvalue mapping option. Let's see a quick working example:

\documentclass{article}
\usepackage{filecontents}

\begin{filecontents*}{\jobname.bib}
@book{fruits,
  title = {The apple and the banana},
  publisher = {Tomatopress},
  editor = {Straw Berry},
  author = {Annoying Orange},
  year = {1970}
}
\end{filecontents*}

\usepackage[utf8]{inputenc}
\usepackage{biblatex}
\addbibresource{\jobname.bib}

\DeclareSourcemap{
    \maps[datatype=bibtex]{
        \map[overwrite]{
            \step[fieldsource=title,
                  fieldset=title,
                  match=\regexp{(\w+)\s+(\w+).*},
                  fieldvalue={$1}
            ]
            \step[fieldset=title,
                  fieldvalue={~$2},
                  append
            ]
        }
    }
}

\begin{document}

Hello world \cite{fruits}.

\printbibliography

\end{document}

The idea here is quite simple: get the first two words of the title key, in my case, $1 = The and $2 = apple. The output is as expected:

Note that in my code I used two \step's: one setting fieldvalue to $1 and the other appending $2 (preceded by a non-breaking space) to it. I was hoping to save a step and try the following code (preferably I'd prefer a breakable space):

\DeclareSourcemap{
    \maps[datatype=bibtex]{
        \map[overwrite]{
            \step[fieldsource=title,
                  fieldset=title,
                  match=\regexp{(\w+)\s+(\w+).*},
                  fieldvalue={$1~$2}
            ]
        }
    }
}

but it doesn't work. I know I could use the replace mapping option - actually, this MWE is too simple - and the expansion of those two variables work, but for the project we are working on, we have to use fieldvalue. I was wondering how can I add an arbitrary number of variables in a fieldvalue all at once instead of incrementally using \step's to append the values.

Any ideas? :)

Answer 1

This is a bug in biber 1.3. Please try version 1.4 from Sourceforge. You'll also need to be using the biblatex 2.4 beta from SourceForge. You might also want to wrap the $1~$2 in \regexp{} to stop biblatex turning the ~ into \nobreakspace {} if it matters to you (the fieldvalue is sanitised specially because it ends up as an XML attribute in the biber .bcf file).