Perl 6 and Parrot Essentials, Second Edition

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 (( . . . ), [ . . . ], or the rule delimiters).
:::	Don’t backtrack over this point; fail out of the current rule or subrule.
<commit>	Don’t backtrack over this point; fail out of the entire match (even from within a subrule).
<cut>	Like <commit>, but also cuts the string matched. The current matching position at this point becomes the new beginning of the string.