How do you search a string on two character sub-string boundaries?

Other than creating a function that examines a string by successive sub-strings or some such thing, does Oracle have a built in function that will examine a string on a boundary specified at runtime? For example, if I have a string 'ABCDEF' and I want to look at two character boundaries (e.g. 'AB' or 'CD' or 'EF' would produce a true result, but not 'BC' or 'DE').