need help checking string is formatted correctly as mac address

hey guys
i have a listbox that imports a text file full of mac address's like so
00:00:00:00:00:00
ff:ff:ff:00:ff:ff
the issue is when someone imports the list like this
000000000000 blah blah
ffffffffffff blah blah
ff:ff:ff:ff:ff:ff blah blah

id there an easy way to recognise this issue and rectify it whilst loading each line into the textbox :)