Put Xpath in variable

Hi all,
this is a piece of my xml tree (node name: field with ( instead of < )

(field id="ViewState.desktop.content.runningActivities.activities.ShowTabs.tabItem.Show.content.ShowFncState.uxToShow.finder.currentFinder.ShowBasicSearchFinder.searchCriteria.criteria.subj.provMetadata.classMetadata" type="text" value="PROVINCIA" mode="read" /)

i need to put the last two words of id (.provMetadata.classMetadata) in a variable... how can i do?

thx a lot for replies

Matteo