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Combined from primary sources listed below.

In Signature literals§

See primary documentation in context for Capture parameters

Prefixing a parameter with a vertical bar | makes the parameter a Capture, using up all the remaining positional and named arguments.

This is often used in proto definitions (like proto foo (|) {*}) to indicate that the routine's multi definitions can have any type constraints. See proto for an example.

If bound to a variable, arguments can be forwarded as a whole using the slip operator |.

sub a(Int $i, Str $s) { say $i.^name ~ ' ' ~ $s.^name }
sub b(|c) { say c.^name; a(|c) }
b(42, "answer");
# OUTPUT: «Capture␤Int Str␤»

In Regexes§

See primary documentation in context for Longest alternation: |

In short, in regex branches separated by |, the longest token match wins, independent of the textual ordering in the regex. However, what | really does is more than that. It does not decide which branch wins after finishing the whole match, but follows the longest-token matching (LTM) strategy.

Briefly, what | does is this:

  • First, select the branch which has the longest declarative prefix.

say "abc" ~~ /ab | a.* /;                 # OUTPUT: «⌜abc⌟␤»
say "abc" ~~ /ab | a {} .* /;             # OUTPUT: «⌜ab⌟␤»
say "if else" ~~ / if | if <.ws> else /;  # OUTPUT: «｢if｣␤»
say "if else" ~~ / if | if \s+   else /;  # OUTPUT: «｢if else｣␤»

As is shown above, a.* is a declarative prefix, while a {} .* terminates at {}, then its declarative prefix is a. Note that non-declarative atoms terminate declarative prefix. This is quite important if you want to apply | in a rule, which automatically enables :s, and <.ws> accidentally terminates declarative prefix.

  • If it's a tie, select the match with the highest specificity.

say "abc" ~~ /a. | ab { print "win" } /;  # OUTPUT: «win｢ab｣␤»

When two alternatives match at the same length, the tie is broken by specificity. That is, ab, as an exact match, counts as closer than a., which uses character classes.

  • If it's still a tie, use additional tie-breakers.

say "abc" ~~ /a\w| a. { print "lose" } /; # OUTPUT: «⌜ab⌟␤»

If the tie breaker above doesn't work, then the textually earlier alternative takes precedence.

For more details, see the LTM strategy.