XI3.0 BPM collect message using substring within correlation

Hi,

I have a requirement to collect a list of IDOCS using the first three characters of a field as a correlation. I tried adding the XPATH expression

substring(/ArticlePriceData/ArticlePrice/Site, 1, 3)

within the correlation editor but this does not work - all IDOCS get added to a single process. Has anybody been able to get this to work? My integration process is based on the SAP example BpmPatternCollectMessage

Many thanks

Ian