In this approach, the pattern-matching that is allowed is restricted to certain semantic categories. A grammar is used to specify the allowable patterns; this allows the restrictions to be specified easily, while allowing more language coverage and easier extension with fewer specified patterns.
Example:
< s> --> what is < ship-property> of < ship> ?
< ship-property> --> the < ship-prop> | < ship-prop>
< ship-prop> --> speed | length | draft | beam
< ship> --> < ship-name> | the fastest < ship2>
| the biggest < ship2> | < ship2>
< ship-name> | Kennedy | Kitty Hawk | Constellation
< ship2> --> < countrys> < ship3> | < ship3>
< ship3> --> < shiptype> < loc> | < shiptype>
< shiptype> --> carrier | submarine | ...
< countrys> --> American | French | British
< loc> --> in the Mediterranean | in the Med | ...
"What is the length of the biggest French sub in the Med?"