InDesign :: Using A Non-marking Subexpression To Find Space Characters

I have section headings in my document that have a Paragraph style applied to them. They are always of the form d.d.d.d( ) follow by a short sentance of text. For eg:
 
5.2.2.1     The rain in Spain falls gently on the crane
 
I am looking for a GREP style that can underline every word character with a character style but not the spaces between the words. It seems to involve nested iterations which I'm not sure how to do including a non-marking expression. Well it aint working how I think it should…
 
5.2.2.1     The rain in Spain falls gently on the crane.

I think this GREP style should get it:  (?<= )((S)+(?:s))+   The positive looks back on the Tab between the last digit of section number and first word but the non-marking subexpressions are not excepting the spaces from underlining, instead I end up with all the Head Text underlines like so:
 
5.2.2.1     The rain in Spain falls gently on the crane