regex_to_add_strong_class.php

<?php
/**
 * Based on the regex findings, we process their response and add the new class to the matching results.
 *
 * @param array $matches An array of matches.
 * @param string The processed content.
 */
function add_extra_strong_class( $matches ) {
	$tag        = $matches[1];
	$attributes = $matches[2];

	// Check if any class attribute exists.
	if ( preg_match( '/\\bclass\\s*=\\s*["\\"]([^"\\"]*)["\\"]/', $attributes, $class_match ) ) {
		$classes = explode( ' ', $class_match[1] );
		// Check if "extra-strong" class already exists.
		if ( ! in_array( 'extra-strong', $classes, true ) ) {
			$classes[] = 'extra-strong';
		}
		$attributes = str_replace( $class_match[0], 'class="' . implode( ' ', $classes ) . '"', $attributes );
	} else {
		// If no class attribute exists, add the entire class attribute.
		$attributes .= ' class="extra-strong"';
	}
	return '<' . $tag . $attributes . '>';
}
preg_replace_callback( '/<(b|strong)([^>]*)>/si', 'add_extra_strong_class', $post_content );