This tells the regexp engine to match either an s or an S at that point.

Have a look at this document for more on regular expressions used in textmate:

Then you also need to change the latex syntax to recognize your (sub)sections as having scope meta.section.latex

In particular, in the LaTeX language file, you will find the line:

begin = '((\\(?:(?:sub)*section|chapter|paragraph|part))(\*?)(?:(\[)[^\[]*?(\]))??(\{))';

near the top.If you change the first "s" there to (s|S) instead, that might do it (haven't tested it).

This will also be matching stuff like \SubSubsection, I hope that's alright.