Get the non-empty element using XPATH

It actually can be simplified as below:

//group/p1[string-length() > 0]    => element text is non-empty
//group/p1[string-length() = 6]    => element text has length 6

This xpath should work as well:

//group/p1[string-length(text()) > 0] 

You may also use [not(node())] Selector. Example: //group/p1[not(node())]

How about something like /root/group/p1[text() and not(../following-sibling::group/p1/text())]

In other words: get the p1 elements that have text and whose group parents are not followed by group nodes that have non-empty p1 elements.