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Message-ID:  <4C5DC2B7.9000004@morningstar2.co.uk>
Date:         Sat, 7 Aug 2010 21:31:51 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: optional (#1:#2) with xparse
To: LATEX-L@listserv.uni-heidelberg.de
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Status: R

On 07/08/2010 14:57, Arno Trautmann wrote:
> Hi all,
>
> I am looking for a way to define an environment that takes a range as an
> optional argument. This should be given in the following way:
>
> \begin{environment}(15:5.23)
>
> with a default value, say (1:3). The two limits should then be parsed as
> single arguments which are separated by the colon.
> I am trying to do this with xparse and got some possible solutions with
> the u and d argument type, but I didn’t manage to get the parentheses
> optional. I guess what I need is some kind of an optional u argument …
>
> Does anyone have a suggestion how to easily implement these requests?
>
> cheers
> Arno
>

Hello Arno,

Parse the argument first as a 'd', then use \SplitArgument to do the 
subsequent work:

\documentclass{article}
\usepackage{xparse}
\NewDocumentEnvironment{example}{>{\SplitArgument{1}{:}}d()}
{}
{
   \IfNoValueTF{#1}
     {#1}
     {\showargs#1}%
}
\NewDocumentCommand\showargs{mm}{Arg 1: #1, Arg 2: #2}
\begin{document}
\begin{example}
First
\end{example}

\begin{example}(1:2)
Second
\end{example}
\end{document}

Here, \SplitArgument will return the result as two groups, unless there 
was \NoValue in which case nothing happens.
-- 
Joseph Wright