Good catch! Updated again for the X and {10}+. `C|\d|P|X` are "legal" costs, tack on to that if you find more. Forgot adventures originally too.

(is:mdfc or is:split or is:adventure) m:/^(.*\/\/ )?(\{(\w\/)?(B|G|C|\d|P|X)+(\/\w)?})+( \/\/.*)?$/

I made it small enough to be (almost) legible and (not quite) user friendly. Think this one is perfect, unless Wizards creates a new mana type.

Change the capital W for other colors. Change + to * to include lands.
(is:mdfc or is:split) m:/^(.*\/\/ )?(\{(\w\/)?(W|C|\d|P)(\/\w)?})+( \/\/.*)?$/

For anyone following, updated value that's more future-proof but still imperfect
(is:mdfc or is:split) m:/(?<!\{[^WC\dP]})(?<!\{[^WC\dP]\/[^WC\dP]})(\{\d\/\w})*(\{(\w\/)?W(\/\w)?})*(\{(\w\/P)})?\{(\w\/)?(W|C|\d|P)(\/\w)?}( \/\/.*)?$/

{G/R}{W}{G/W} matches that and shouldn't, but close otherwise!

Updated it a bit more. Should return all current cards, but doesn't account for some mana combos that don't exist yet. Changing the capital Ws to your color of choice will work for different colors, and don't ask for two colors lol
is:mdfc m:/(?<!\{[^WC\d]})\{(\w\/)?(W|C|\d|P)(\/\w)?}( \/\/.*)?$/

Have to refine this a bit as there's some pieces overlooked like colorless sides, weird mana variants, etc. Expect to come back later to complete it!
Unfortunately though this basically only works with casting costs.

`is:mdfc m:/(?<!\{(R|B|G|U)})\{(\w\/)?W(\/\w)?}( \/\/.*)?$/`