> I think I can distinguish :: from ::: - :: fails the current branch point,
> whereas ::: fails the entire rule.
Correct.
> I think I can distinguish between ::: and <commit>; the implementation
> is a bit tricky, because ::: fails the current match - easy enough -
> whereas <commit> has to specify to all upper-level matches that they
> failed too. Is it reasonable for failing a <commit> to be an exception?
It's not an exception, but you could certainly *implement* it that way.
> I don't think I can distinguish between : and ::. Essentially, how many
> nodes does each commit to? I would expect the following
>
> [ a+ : b | c+ : d | e+ : f ]
>
> to do precisely the same as
>
> [ a+ :: b | c+ :: d | e+ :: f ]
It doesn't. The first says:
[ # Non-capturingly...
a+ # Match 1+ a's
: # And never backtrack to match fewer a's
b # Then match a b
| # Or if that failed...
c+ # Match 1+ c's
: # And never backtrack to match fewer c's
d # Then match a d
| # Or if that failed...
e+ # Match 1+ e's
: # And never backtrack to match fewer e's
f # Then match an
]
whereas the second says:
[ # Non-capturingly...
a+ # Match 1+ a's
:: # And if you ever have to backtrack,
# then immediately fail the entire surrounding group
b # Then match a b
| # Or if that failed...
c+ # Match 1+ c's
:: # And if you ever have to backtrack,
# then immediately fail the entire surrounding group
d # Then match a d
| # Or if that failed...
e+ # Match 1+ e's
:: # And if you ever have to backtrack,
# then immediately fail the entire surrounding group
f # Then match an f
]
So a single colon has the same effect as the Perl 5 (?>...) metasyntax.
> I don't see why Larry describes one as "if-then", but not the other.
> Basically, I don't think I'm sure what : "gives up".
It gives up tracktracking the immediately preceding atom.
The behaviour is explained quite well in E5 I think.
Damian
