Collating characters?

[Art Hixson]

I'm just trying out gawk for the first time.  Forgive me if I'm missing 
something.  I'm running on XP (darn it).

I'm trying to use the construction /[[.70.]]/ to treat this character 
pair as a unit.  However, contrary to the documentation, it gives :

*gawk: prg.txt:7: fatal: Invalid collation character: /[[.70.]]/*

when I try just /[.70.]/ I get all the 7s and 0s in any order , i.e., 
ORed., which is just a character list.

Where have I gone astray?  (leaving out my personal life as that would 
take too many pages)

--Art