The following are treated as valid digraphs when used as a collating name:

"ae", "Ae", "AE", "ch", "Ch", "CH", "ll", "Ll", "LL", "ss", "Ss", "SS", "nj", "Nj", "NJ", "dz", "Dz", "DZ", "lj", "Lj", "LJ".

So for example the expression:

[[.ae.]-c] 

will match any character that collates between the digraph "ae" and the character "c".