How do I check for specific shapes in a tilemap?

I'm working on a prototype for a game. The player has the ability to place 16 X 16 blocks on the map, using the "tilemap" object. I want to be able to have the player drawing a specific shape in a non specific area trigger an event. For instance if the player drew the letter "H" in blocks anywhere on the screen, he would trigger an event. I hope all of you get what I'm saying. Thank you in advance

This plugin might come in handy..

?

I don't think this is what i'm looking for. Is there any way I can do this using the Tilemap object? also, The player is placing blocks on the screen, not actually drawing like that.

Ah, ok..

Misunderstood..

I guess putting the information about the blocks in an array and checking if the shape is the shape you want on tile changed would be the way..

This is just theory, because for me it would be too difficult to create this..

Ok, Thanks for responding