- publishing free software manuals
Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

Sales of this book support The Perl Foundation! Get a printed copy>>>

4.11.4 Smart matching in detail

The behaviour of a smart match depends on what type of thing its arguments are. The behaviour is determined by the following table: the first row that applies determines the match behaviour (which is thus mostly determined by the type of the right operand). Note that the smart match implicitly dereferences any non-blessed hash or array ref, so the "Hash" and "Array" entries apply in those cases. (For blessed references, the "Object" entries apply.)

Note that the "Matching Code" column is not always an exact rendition. For example, the smart match operator short-circuits whenever possible, but grep does not.

$a $b Type of Match Implied Matching Code
Any undef undefined !defined $a
Any Object invokes ~~ overloading on $object, or dies
Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
Any CodeRef scalar sub truth $b->($a)
Hash Hash hash keys identical (every key is found in both hashes)
Array Hash hash keys intersection grep { exists $b->{$_} } @$a
Regex Hash hash key grep grep /$a/, keys %$b
undef Hash always false (undef can't be a key)
Any Hash hash entry existence exists $b->{$a}
Hash Array hash keys intersection grep { exists $a->{$_} } @$b
Array Array arrays are comparable[2]
Regex Array array grep grep /$a/, @$b
undef Array array contains undef grep !defined, @$b
Any Array match against an array element[3] grep $a ~~ $_, @$b
Hash Regex hash key grep grep /$b/, keys %$a
Array Regex array grep grep /$b/, @$a
Any Regex pattern match $a =~ /$b/
Object Any invokes ~~ overloading on $object, or falls back:
Any Num numeric equality $a == $b
Num numish[4] numeric equality $a == $b
undef Any undefined !defined($b)
Any Any string equality $a eq $b
  1. empty hashes or arrays will match.
  2. that is, each element smart-matches the element of same index in the other array. [3]
  3. If a circular reference is found, we fall back to referential equality.
  4. either a real number, or a string that looks like a number
ISBN 9781906966027Perl Language Reference ManualSee the print edition