in reply to perl look ahead regular expression that is optional?
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
- 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'
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?
Alex / talexb / Toronto
Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.