问题
The docs for Perl 6 longest alternation in regexes punt to Synopsis 5 to document the rules for longest token matching. There are three rules if different alternatives would match a substring of the same length:
- The longest declarative prefix breaks the tie
- The highest specificity breaks the tie
- "If it's still a tie, use additional tie-breakers."
- The left most alternation finally wins
It's that third rule that I'm curious about.
回答1:
First the way the text is organized makes clear that the behaviour of the implementation must be deterministic (not random).
Second - and more important - describing the exact behaviour of existing implementations could fill an entire, hard-to-understand page as every corner case has to be described. In addition such a specification would limit degrees of freedom of the implementation. Let's assume some implementation supports a "fastest implementation" flag. Such an implementation can use unspecified parts to make short-cuts. So leaving the behaviour unspecified resp. restricted to the minimum has some advantages.
来源：https://stackoverflow.com/questions/49943078/whats-an-additional-tie-breaker-for-perl-6-longest-token-matching