This is an interesting question -- I like brain-teasers as much as the next developer, so I wrote a solution. Except it doesn't work, because I think the example you have is faulty. Perhaps someone else can find the flaw in my reasoning.

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.

OK -- foobar, then optionally (zero or one 'a', 'b', 'c'), then optionally ('X' followed by zero or one 'a', 'b', 'c'). But then there's a list of examples, and #5 seems to break the rule about the second group ..

foobarabcX        must not match, nothing follows 'X'
[download]

I'm not sure this is right. The second optional group has been defined as 'X', followed by zero or one 'a', 'b' and 'c'. Which means I could have an 'X' followed by zero 'a', 'b' and 'c'. Which means that foobarabcX is a valid pattern.

Anyone else see the flaw?

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