Sorry, I won't be much help. the PHPBB search is limited (and buggy too)
My recommendation is to the use the google search but I understand that sometimes you have use the native search, which is not very good, and that's why we installed the Google search
Founder of GMAT Club
Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
Need GMAT Book Recommendations? Best GMAT Books
Co-author of the GMAT Club tests
Have a blog? Feature it on GMAT Club!