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Language documentation: Operators §

From Operators

(Operators) infix ff §

sub infix:<ff>(Mu $a, Mu $b)

Also called the flipflop operator, compares both arguments to $_ (that is, $_ ~~ $a and $_ ~~ $b). Evaluates to False until the left-hand smartmatch is True, at which point it evaluates to True until the right-hand smartmatch is True.

In effect, the left-hand argument is the "start" condition and the right-hand is the "stop" condition. This construct is typically used to pick up only a certain section of lines. For example:

my $excerpt = q:to/END/; 
Here's some unimportant text.
=begin code
    This code block is what we're after.
    We'll use 'ff' to get it.
=end code
More unimportant text.
END
 
my @codelines = gather for $excerpt.lines {
    take $_ if "=begin code" ff "=end code"
}
# this will print four lines, starting with "=begin code" and ending with 
# "=end code" 
say @codelines.join("\n");

After matching the start condition, the operator will then match the same $_ to the stop condition and act accordingly if successful. In this example, only the first element is printed:

for <AB C D B E F> {
    say $_ if /A/ ff /B/;  # OUTPUT: «AB␤» 
}

If you only want to test against a start condition and have no stop condition, * can be used as such.

for <A B C D E> {
    say $_ if /C/ ff *;    # OUTPUT: «C␤D␤E␤» 
}

For the sed-like version, which does not try $_ on the stop condition after succeeding on the start condition, see fff.

This operator cannot be overloaded, as it's handled specially by the compiler.