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

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

##</code><code>##

sub {
  /\G (?:
      ... (?{ code1(...); })  # pattern 1, handler for pattern 1
    | ... (?{ code2(...); })  # pattern 2, handler for pattern 2
    | ... (?{ code3(...); })  # and so on
  )/gcx;
}

##</code><code>##

sub parse {
    my $input= shift;
    my $code= ... # assemble regex sub text like above
    my $lexer= eval $code
        or die "BUG: syntax error in generated code: $@";
    local $_= $input;
    &$lexer || die "Syntax error at '" . substr($_, pos, 10) . "'"
      while pos < length;
}