You may find this useful:  https://github.com/filiph/linkcheck

On Monday, May 24, 2021 at 7:56:48 AM UTC-7 Gwern Branwen wrote:

> When writing long complicated Pandoc documents over years, it is easy
> for section or anchor self-links to drift and become invalid.
> Pandoc/hakyll will not warn you about links being broken, they do not
> break in any visible ways or affect validation, and it's surprisingly
> difficult to get any linkchecking tool to warn you about them
> specifically. (The W3C linkchecking tool has no option to check only
> anchors and insists on checking all external links, while
> 'linkchecker' has a plugin for this I am told is broken and also no
> way to avoid checking all external links - this renders both tools
> completely infeasible for regularly checking a website the size of
> gwern.net, where a linkchecker run may take several days to finish.)
> This is despite being really quite a simple task: get all hrefs
> starting with '#', check that an ID corresponds, print out any ones
> missing.
>
> dbohdan wrote a PHP script for me to check gwern.net pages, which has
> worked well and exposed at least 20 erroneous anchors I've fixed, and
> which is fast enough to include in the site sync script so new errors
> will be picked up immediately. This may be useful to other
> Pandoc/hakyll users.
>
> Source: 
> https://github.com/gwern/gwern.net/blob/master/build/anchor-checker.php
>
> #! /usr/bin/env php
> <?php
> // Check anchors in HTML files. Only checks anchors local to each document.
> // Anchors prefixed with a filename are ignored even if they refer to the
> // same file. Anchors with no element with the corresponding fragment ID
> // are written to stderr prefixed with the filename.
> //
> // Usage: anchor-checker.php [FILE]...
> //
> // To the extent possible under law, D. Bohdan has waived all copyright and
> // related or neighboring rights to this work.
> //
> // Date: 2021-05-24.
> // Requirements: PHP 7.x with the standard DOM module.
>
> error_reporting(E_ALL);
>
> function main($files) {
> $exit_code = 0;
>
> foreach ($files as $file) {
> $bad_anchors = check_file($file);
> foreach ($bad_anchors as $a) {
> fprintf(STDERR, "%s\t%s\n", $file, $a);
> $exit_code = 1;
> }
> }
>
> exit($exit_code);
> }
>
> function check_file($file) {
> $html = file_get_contents($file);
> // An ugly hack to get around missing HTML5 support tripping
> up the parser.
> $html = preg_replace("/<wbr>/", "", $html);
>
> if (preg_match("/^\s*$/", $html)) return [];
>
> $dom = new DOMDocument();
> $dom->loadHTML($html, LIBXML_NOERROR | LIBXML_NOWARNING);
>
> return check_document($dom);
> }
>
> function check_document($dom) {
> $ids = (new DOMXpath($dom))->query("//@id");
> $id_set = array();
>
> foreach ($ids as $id) {
> $id_set["#" . $id->value] = true;
> }
>
> $bad_anchors = array();
> $hrefs = (new DOMXpath($dom))->query("//a/@href");
> foreach ($hrefs as $href) {
> $value = trim($href->value);
>
> if (substr($value, 0, 1) !== "#") continue;
>
> if (!array_key_exists($value, $id_set)) {
> $bad_anchors[] = $value;
> }
> }
>
> return $bad_anchors;
> }
>
> main(array_slice($argv, 1));
>
> This can be used as a post-compilation check like
> `static/build/anchor-checker.php ./_site/"$HTML"` or what have you.
>
> -- 
> gwern
> https://www.gwern.net
>

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