Highlighting keywords with spaces with the listings package

Question

Possible Duplicate:
How to emphasize within a listing two successive identifiers separated by a space?

I am trying to highlight the following keywords with the listings package:

over window:time

over window:length

If I declare:

\lstdefinelanguage{drl}{
  morekeywords=[2]{length,over,time,window},
  keywordstyle=[2]\color{darkred}\bfseries
}

I get the job done, however the colon is not highlighted and I cannot use length, over, time and window as identifiers or otherwise they are highlighted as keywords (time, for example, is often used as identifier in my language since I have to deal with temporal events and operators).

So I tried the following:

\lstdefinelanguage{drl}{
  alsoletter={:},
  morekeywords=[2]{over,window:length,window:time},
  keywordstyle=[2]\color{darkred}\bfseries
}

and now window:length and window:time are properly recognised as a single entity but still I cannot user over as an identifier. So I tried to go further on that way by declaring the following (comments represent additional unsuccessful tries):

\lstdefinelanguage{drl}{
  alsoletter={ ,:},
  % alsoletter={\ ,:},
  % alsoletter={{ },:},
  morekeywords=[2]{over window:length,over window:time},
  keywordstyle=[2]\color{darkred}\bfseries
}

How are blankspaces encoded? What am I missing? What am I doing wrong? Any help is much appreciated! Below you can find a full working example if you want to play with the code:

\documentclass{article}
\usepackage{xcolor}
\usepackage{listings}

\definecolor{darkred}{RGB}{151,0,11}

\lstdefinelanguage{drl}{
  alsoletter={ ,:},
  % alsoletter={\ ,:},
  % alsoletter={{ },:},
  morekeywords=[2]{over window:length,over window:time},
  keywordstyle=[2]\color{darkred}\bfseries
}

\lstset{
  gobble=4,
  language=drl
}

\begin{document}
  \begin{lstlisting}
    over = 5 s;
    time = 10 ms;
    eval(time < over)
    this over window:length( 5 )
    this over window:time( 30s )
  \end{lstlisting}
\end{document}

Thanks in advance!