请教一个模式匹配的问题

eric.xie

my $x='Perl is free.';
my $y='ManagerWare costs $99.95.';
foreach ($x,$y) {
        /^(\w+) (?:is|(costs)) (?(2)(\$\d+)|\w+)/;
        if($3) {
                print "$1 costs money.\n";
        } else {
                print "$1 doesn't cost money.\n";
        }
}
上面这个例子中，模式?(2)这个是什么意思？

zhlong8

perlre 摘录

(?(condition)yes-pattern|no-pattern)
(?(condition)yes-pattern)

Conditional expression. Matches yes-pattern if condition yields a true value, matches no-pattern otherwise. A missing pattern always matches.

Here's a summary of the possible predicates:

    (1) (2) ...

    Checks if the numbered capturing group has matched something.
    (<NAME>) ('NAME')

    Checks if a group with the given name has matched something.
    (?=...) (?!...) (?<=...) (?<!...)

    Checks whether the pattern matches (or does not match, for the '!' variants).
    (?{ CODE })

    Treats the return value of the code block as the condition.

eric.xie

多谢版主指点！