On 3/7/06, Gavin Bowlby <[EMAIL PROTECTED]> wrote: > So > > (\.)? > > differs from > > (\.?) > > in that there is no match in the first case, but there is a match in the > second case?
Well, to be sure, we're talking about a successful pattern match in either case. But the first one has a quantifier applied to memory parentheses, and the result of that isn't easy to intuit. Quantifiers, such as *, ?, and +, indicate that the preceding item is repeated a certain number of times. They are the "loops" we use in programming the regular expression engine. But if memory parentheses are involved, which iteration of the quantifier gets remembered? It's the last iteration, generally. (There are complexities. Curious folks should see the perl docs and Friedl's "Mastering Regular Expressions".) But if the key part of the pattern is matched zero times, as the ? allows, there's no value for the corresponding memory variable, so it gets undef. The moral of the story is: Don't use quantifiers on memory parentheses. It will probably confuse the next programmer. :-) But it's good to be prepared for memory variables to be undefined, even after a successful match, if the pattern allows for them to be skipped. > I'm still confused as to why the placement of the ? operator inside or > outside the parentheses makes a difference. > > I thought the parentheses were only there to bind the result to a $N > variable, not to change the operations of the pattern matching itself. Ah, parentheses in pattern matches have two functions: They group items together, as in mathematics; and they activate the regular expression engine's memory. If you wish only grouping, Perl's patterns also offer non-memory parentheses. But ordinary parentheses do both jobs. Another way to think about it: (\.)? is "an optional memory (of a dot)", while (\.?) is "a memory of an optional dot". Cheers! --Tom Phoenix Stonehenge Perl Training -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] <http://learn.perl.org/> <http://learn.perl.org/first-response>
