#!/usr/bin/env perl

use 5.014;
use warnings;

my @html = (
    '<h2>2.1. Match Literal Text</h2>',
        '<h3>2.1.1 H3 Heading</h3>',
            '<h4>2.1.1.1 H4 Heading</h4>',
    '<h2>2.2. Match Nonprintable Characters</h2>',
    '<h2>2.3. Match One of Many Characters</h2>',
);

my $re = qr{^(<h[1-6])(>[1-6. ]+)([^<]+)(</h[1-6]>)$};

for my $i (0 .. $#html) {
    $html[$i] =~ s{$re}{$1 . ' id="' . lc($3) =~ y/ /_/r . "\"$2$3$4"}e;
}

say for @html;