Respond to Chat commands

[Lanodar]

Hey

I'm writing an Addon to organize Baradin Hold Classruns.

Players have to send !class role per whisper and will be invited if their slot is available.

If someone writes !druid tank , !druid healer or !druid DPS, they'll receive a whisper and will be invited.

But if someone writes !DRUID TANK or !warlock healer. nothing will happen because those commands aren't defined.

How can i realize that if someone posts !warlock healer, there will be an answer like "Undefined command. DPS Classes just need to post ther Class (!warlock,!rogue,!mage)"

Or if Someone posts "!DROOD HEALER! "Undefined command, please use !druid healer" in lower case letters)