Ocamllex - What is the difference between characters ? ( # )

Question

They've an operator with ocamllex which is the #: difference between two characters or character sets.

Here, there is a notion I don't understand: it is the difference between characters. What does mean the difference between characters? So if someone can explain me, I take it!

Answer 1

[ 'a' - 'z' ] # [ 'a' 'e' 'i' 'o' 'u' ] denotes the set of all characters between a and z that are not a, e, i, o or u. More generally, foo # bar denotes the set of elements that are in foo but not in bar.