Grouping on BI Publisher RTF Template by substring value

Hello,

Is there a way to group records by substring value of a field? Is it even possible?

For e.g. if there are multiple lines for a sales order with line numbers 1.000, 1.001, 2.000: we want to group 1.000 and 1.001 together. So output should show two lines instead of three.

I tried <?for-each-group:Detail_Line_1_Section_S2;./substring-before(BIPLineNumber_lnid_ID298, '.')?>

But it did not work as expected. If the XML has 1.000 and 1.001 it grouped them correctly into 1 record, but when the XML has 1.000 and 2.000 that is also getting grouped into 1 line when it should show as two lines.

Appreciate any help!