Auto-tag Amazon.com links (v2: Clean)

auto-tag-amazon-links-v2.php

<?php
/**
 * Auto-tag Amazon.com links (v2: Clean)
 * 
 */
add_filter( 'the_content', 'ac_auto_tag_amazon_links_v2' );

function ac_auto_tag_amazon_links_v2( $content ) {

	// Variables
	$amazon_tag = array( 'tag' => 'adamcapr-20', ); // Update with your tracking ID

	// Reconstruct /dp/ASIN URLs to remove all extraneous strings and parameters; add tag to URLs

	// Find all Amazon URLs
	preg_match_all( '/a href="(https?:\/\/(?:www)?\.amazon\.com[^"]*)"/', $content, $matches );

	// Find all /dp/ASINs
	$asins = array();
	foreach ( $matches[1] as $amazon_url ) {
		preg_match( '/(?:\/dp\/([A-Z0-9]{10}))/', $amazon_url, $asins[] );
	}
	foreach ( $asins as $asin ) {
		$matches[2][] = ( isset( $asin[1] ) ) ? $asin[1] : '';
	}

	// Reconstruct URLs
	$search_urls = $replace_urls = array();
	foreach ( $matches[1] as $key => $amazon_url ) {
		// Search URLs
		$search_urls[] = $amazon_url;
		
		// Replace URLs: No /dp/ASIN
		if ( empty( $matches[2][ $key ] ) ) {
			$replace_urls[] = add_query_arg( $amazon_tag, $amazon_url );
		}
		// Replace URLs: /dp/ASIN
		else {
			$replace_urls[] = add_query_arg( $amazon_tag, 'https://www.amazon.com/dp/' . $matches[2][ $key ] . '/' );
		}
	}

	// preg_replace
	foreach ( $search_urls as $key => $search_url ) {
		$content = preg_replace( '/a href="' . preg_quote( $search_url, '/' ) . '"/', 'a href="' . esc_url( $replace_urls[ $key ] ) . '"', $content );
	}

	return $content;

}