Need help with forward looking regular expression. See if you can write an expression that satisfies this spec: 
Example in pseudo code:
   find a string 'foobar' that is followed by two sets of optional 
   characters, the first set of characters might contain a b or c,
   in that order. Only zero or one a, zero or one b, and zero or one c is allowed.
   The second set of characters is always preceded by an X (to delimit
   that this is the second set) and might contain a b or c,
   Only zero or one a, zero or one b, and zero or one c is allowed in the second set.

   When the second set is present, it is always prececeded by an 'X':

   foobarfirst set of 'a','b',or 'c' X second set of 'a','b',or 'c'

input:

foobar            matches, first set is empty, no X, no second set.
foobarabc         matches, first set contains abc, no X, no second set.
foobaraXa         matches, first set contains a, X delimiter, second set contains a.
foobarabcXabc     matches, first set contains abc, X delimiter, second set contains abc.

foobarabcX        must not match, nothing follows 'X'
foobarabca        must not match, no 'X' precedes last 'a'.


In perl, matching up to the 'X' is easy:
   m/foobar(a)?(b)?(c?)X?/

But matching the next bit is tough.  I've tried forward look/backward look assertions
but can't nail the regexp I want.  Adding to the difficulty is that all options in
the first second, and all options in the second set, must be 'grouped' so I can 
retrieve the values from the expression.

In reply to perl look ahead regular expression that is optional? by bigsipper

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