It might be useful to list here the operators that work:
= - matches exact value
=*xxx - matches values ending xxx
=xxx* - matches values beginning xxx
=*xxx* - matches values containing xxx
=* - matches all values (if set - NULLS are not returned)
>=xxx - matches everthing from xxx to end of directory
<=xxx - matches everything up to xxx in directory
~=xxx - matches similar entries (not all systems)
Boolean operators for constructing complex search
&(term1)(term2) - matches term1 AND term2
| (term1)(term2) - matches term1 OR term2
!(term1) - matches NOT term1
&(|(term1)(term2))(!(&(term1)(term2)) - matches XOR term1 term2
some of the more compelx constructions seem to work with varying degrees of efficiency - sometimes it can be better to filter some of the results with the search and do further filtering in PHP.