Linux Admin 3: regular expressions 1

[ ] matches sets of characters.
so [123] will match 1, 2 OR 3 in a single position
[tf]in will match 'tin' or 'fin'
[^] means NOT.
so ^[^4] means any line that doesn't START with a 4.
[^t] means any character that isn't a t in it
[^t]h means character that isn't a t, followed by an h
Using NOT is...tricky at best. Try to avoid it.