InfoPath Dev

Hi, I'm new to xpath and having a problem creating the query I need.

I have an xml document as a secondary datasource. I would like to use a text string that the user types in a text box to search for all matching nodes in the xml datasource. I have tried the 'contains' and 'starts-with' xpath functions but neither one seems to work if you pass the search string argument as a string variable.

Is there a better way to achieve the same results? Is there another xpath function I should be using?

Thanks.

Here is a bit more detail:

I get the text that the user typed and put it in a string variable:

string searchString = thisXDocument.DOM.selectSingleNode("//SIUData/ReportContacts/last_name").text;

Then I pass the string variable to an xpath function to find all nodes containing that string:

contactNodes = thisXDocument.GetDOM("Contacts").selectNodes("ContactData/Contact[contains(last_name, searchString)]");

And then I get the node count:
nodeCount = contactNodes.length;

If I substitute a literal string for the searchString variable, everything works fine. Is there any way I can use a string variable to search an xml document using xpath an query??

Found the answer, and it is very basic (I lost sight of the fact that the whole 'selectNodes' is taking a string):

Instead of:

contactNodes = thisXDocument.GetDOM("Contacts").selectNodes("ContactData/Contact[contains(last_name, searchString)]");

I needed to do the following:

contactNodes = thisXDocument.GetDOM("Contacts").selectNodes("ContactData/Contact[contains(last_name,'" + searchString + "')]");