On 5/12/05, Patrick R. Michaud <[EMAIL PROTECTED]> wrote: > I have a couple of questions regarding C< :: > in perl 6 rules. > First, a question of verification -- in > > $rule = rx :w / plane :: (\d+) | train :: (\w+) | auto :: (\S+) / ; > > "travel by plane jet train tgv today" ~~ $rule > > I think the match should fail outright, as opposed to matching "train tgv". > In other words, it acts as though one had written > > $rule = rx :w / plane ::: (\d+) | train ::: (\w+) | auto ::: (\S+) / ; > > and not > > $rule = rx :w /[ plane :: (\d+) | train :: (\w+) | auto :: (\S+) ]/ ;
Those both do the same thing (which is the same as your example). When you fail over the :: after plane, it skips out of the alternation looking for something to backtrack before it. Since there is nothing, the rule fails. > Does this sound right? > > Next on my list, S05 says "It is illegal to use :: outside of > an alternation", but A05 has > > /[:w::foo bar]/ > > which leads me to believe that :: isn't illegal here even though there's > no alternation. I'd like to strike that sentence from S05 Yeah, I think using :: to break out of the innermost bracketing group is helpful even without an alternation present. > Also, A05 proposes incorrect alternatives to the above > > /[:w[]foo bar]/ # null pattern illegal, use <null> > /[:w()foo bar]/ # null capture illegal, and probably undesirable > /[:w\bfoo bar]/ # not exactly the same as above > > I'd like to remove those from A05, or at least put an "Update:" > note there that doesn't lead people astray. One option not > mentioned in A05 that we can add there is > > /[:w<?null>foo bar]/ > > which is admittedly ugly. > > So, now then, on to the item that got me here in the first place. > The upshot of all of the above is that > > rx :w /foo bar/ > > is not equivalent to > > rx /:w::foo bar/ Yeah, but it is. So no problem. :-) > which may surprise a few people. The :: at the beginning of > the pattern effectively anchors the match to the beginning of > the string or the current position -- i.e., it eliminates the > implicit C< .*? > at the start of the match. Ohhh, ohh. There isn't an implicit .*? at the beginning of the match. It's more like there's an implicit .*? followed by a rule call to the match. Think of it as that we're trying to match the pattern at any position rather than there being an implicit .*?. Luke
