Add Amazon affiliate tag to all Amazon links in WordPress the_content

amazon-links.php

<?php
// This adds a hard coded Amazon affiliate tag to all amazon links in the_content
// Code adapted from the abandoned "Amazon Affiliate Tag" plugin
// https://wordpress.org/plugins/amazify/ by Maxime Bridon https://bridon.fr/


function add_amazon_tag( $content ) {
	// the affiliate tag for Peter Bregman
	$affiliate_tag = 'yourtaghere-123';
	// regex to find any link
	$link_regex = "<a\s[^>]*href=(\"??)([^\" >]*?)\\1[^>]*>(.*)<\/a>";
	// regex to find amazon urls
	$amazon_regex = "(.*)amazon\.(com|co.uk|de|fr|co.jp|ca)+";
	// find all the links
	if ( preg_match_all( "/$link_regex/siU", $content, $link_matches, PREG_SET_ORDER ) ) {
		// loop through the links
		foreach ($link_matches as $match) {
			// if it is an Amazon link...
			if ( preg_match( "/$amazon_regex/i", $match[2] ) ) {
				// get the URL of the amazon link, parse it into component parts
				$thelink = str_replace('&#038;', "&amp;", $match[2]);
				$parsed = parse_url($thelink);

				if ( isset( $parsed['query'] ) && preg_match( '#tag#i', $parsed['query'] ) ) {
					// if the link has a tag already, replace it
					$query_string = html_entity_decode($parsed['query']);
					parse_str($query_string, $variables);
					$variables["tag"] = $affiliate_tag;
					$new_query = http_build_query($variables, '', '&amp;');
					$newlink = '<a href="'.$parsed['scheme'].'://'.$parsed['host'].$parsed['path'].'?'.$new_query.'">'.$match[3].'</a>';
				} elseif ( isset($parsed['query'] ) ) {
					// or if the link has a query string, but no tag - append our tag to the query
					$newlink = '<a href="'.$thelink.'&amp;tag='.$affiliate_tag.'">'.$match[3].'</a>';
				} else {
					// otherwise, the link has no tag or query - just add our tag as a new query
					$newlink = '<a href="'.$thelink.'?tag='.$affiliate_tag.'">'.$match[3].'</a>';
				}
				// replace the old link with the newly contstructed one
				$content = str_replace($match[0], $newlink, $content);
			}
		}
	}
	return $content;
}

add_filter('the_content', 'add_amazon_tag');