Programming Perl, 4th Edition

Smartmatch Operator

First available in v5.10.1,^[63] binary ~~ does a “smartmatch” between its arguments. This is mostly used implicitly in the when construct, although not all when clauses call the smartmatch operator. Unique among all of Perl’s operators, the smartmatch operator can recurse.

It is also unique in that all other Perl operators impose a context (usually string or numeric context) on their operands, autoconverting those operands to those imposed contexts. In contrast, smartmatch infers contexts from the actual types of its operands and uses that type information to select a suitable comparison mechanism.

The ~~ operator compares its operands “polymorphically”, determining how to compare them according to their actual types (numeric, string, array, hash, and so on). Like the equality operators with which it shares the same precedence, ~~ returns 1 for true and "" for false. Much like the =~ binding operator, this operator’s right argument is considered to be a pattern that either accepts or rejects the left argument. However, the notion of “pattern” is generalized greatly, and nearly any value can function as a pattern, or as a list of patterns.

So ~~ is often best read aloud as “matches” or “matches any of”, because the left operand submits itself to be accepted or rejected by the right operand (or some part of the right operand).

The behavior of a smartmatch depends on what type of things its arguments are, as determined by Table 3-7. The first row of the table whose ...