There may have been many similar attempts, but come to think of it,
adding a simple higher-order rule to OMeta that applies the given rule
for a given number of times is just straightforward.  For example, if
you want to only parse a string that represents fully padded
hexadecimal digits for a 8 or 4 byte quantity, you can write:

hex16or8 =
  numberOf(#hex. 16)
| numberOf(#hex. 8)

where #hex is something like:

hex :c =
   ?[(c between $0 and: $9) or: [c between: $A and: $F]


The numberOf rule looks like:

numberOf :rule :count =
    [0]:c [OrderedCollection new]:xs
    ([c+1]:c ?[c <= count] apply(rule):x [xs add: x])*
    ?[c = (count + 1)] -> [xs]

--
-- Yoshiki