《perl语言入门》疑问求教

QUOTE:

Instead of remembering numbers such as $1, Perl 5.10 lets us name the captures directly in the regular expression. It saves the text it matches in the hash named %+: the key is the label we used and the value is the part of the string that it matched. To label a match variable, we use (?<LABEL>PATTERN) where we replace LABEL with our own names.* We label the first capture name1 and the second one name2, and look in $+{name1} and
$+{name2} to find their values:
use 5.010;
my $names = 'Fred or Barney';
if( $names =~ m/(?<name1>\w+) (?:and|or) (?<name2>\w+)/ ) {
say "I saw $+{name1} and $+{name2}";
}

QUOTE:

Now that we have a way to label matches, we also need a way to refer to them for back
references. Previously, we used either \1 or \g{1} for this. With a labeled group, we can
use the label in \g{label}:
use 5.010;
my $names = 'Fred Flinstone and Wilma Flinstone';
if( $names =~ m/(?<last_name>\w+) and \w+ \g{last_name}/ ) {
say "I saw $+{last_name}";
}
We can do the same thing with another syntax. Instead of using \g{label}, we use
\k<label>:†
use 5.010;
my $names = 'Fred Flinstone and Wilma Flinstone';
if( $names =~ m/(?<last_name>\w+) and \w+ \k<last_name>/ ) {
say "I saw $+{last_name}";
}