June 2004
Intermediate to advanced
298 pages
12h 19m
English
Content preview from Perl 6 and Parrot Essentials, 2nd Edition
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Backtracking Control
Backtracking is triggered
whenever
part of the pattern fails to match. You can also explicitly trigger
backtracking by calling the
fail
function within a closure. Table 7-8 shows some metacharacters and
built-in rules relevant to backtracking.
Table 7-8. Backtracking controls
|
Operator |
Meaning |
|---|---|
|
: |
Don’t retry the previous atom; fail to the next earlier atom. |
|
|
Don’t backtrack over this point; fail out of the
closest enclosing group ( |
|
|
Don’t backtrack over this point; fail out of the current rule or subrule. |
|
|
Don’t backtrack over this point; fail out of the entire match (even from within a subrule). |
|
|
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