How do l clean a field that has funny characters. l want to remove anything that is not numeric and if the lenght of the numeric characters is less than 13 pad it with zeros?

i.e 234rfg!*23 and just remain with 23423 then pad it with zeros to make it 13 characters 0000000023423.

So the first thing l would like to do is to check if its numeric if so check the length and make sure its 13 long. How do l do that