examples of what should be searchable:

1. cost includes "WW"
2. does not include "RRR" (coupled with red card filter for example)

3. includes "2G" (across hybrid - would find Messenger Falcons)
4. cost has 2 or more hybrid symbols
5. cost has 4 mana symbols of the same color

etc.

Actually, this seems to be quite simple, see magiccards.info:
they have "Mana Cost: =/</>/<=/>= (XYZ 0-9 WUBRG) "
but it seems it can't negations or 4. and 5.