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For example, $+{foo} is equivalent to $1 after the following match:

    'foo' =~ /(?<foo>foo)/;

sub {
  /\G (?:
...
    | ... (?{ code3(...); })  # and so on
  )/gcx;
}

sub parse {
    my $input= shift;
...
    &$lexer || die "Syntax error at '" . substr($_, pos, 10) . "'"
      while pos < length;
}