Alter output of WordPress TinyMCE field content

markup.php

<?php
/**
 * Add spans and icons to buttons added by applying custom link classes in the WYSIWYG editor
 * @param $content
 * @return string
 */
function wapr_add_icons_to_buttons($content): string {
    return preg_replace_callback('/<a class="btn btn--(?:.*) btn--icon" href="(?:.*)">(.*)<\/a>/', function($match) {
        return str_replace($match[1], '<span>'.$match[1].'</span><i class="fa-regular fa-chevron-right"></i>', $match[0]);
    }, $content);
}
add_filter('the_content', 'wapr_add_icons_to_buttons');
add_filter('acf_the_content', 'wapr_add_icons_to_buttons');

tinymce.php

<?php
/**
 * Add custom formats menu to TinyMCE
 * @param $buttons
 * @return array
 */
function wapr_add_mce_styleselect($buttons): array {
	array_unshift($buttons, 'styleselect');

	return $buttons;
}
add_filter('mce_buttons_2', 'wapr_add_mce_styleselect');


/**
 * Populate custom formats menu in TinyMCE
 * Notes: 'selector' for block-level element that format is applied to; 'inline' to add wrapping tag e.g.'span'
 *         Using 'attributes' to apply the classes instead of 'class' ensures previous classes are replaced rather than added to
 * @param $init_array
 *
 * @return array
 */
function wapr_add_mce_styles($init_array): array {
	$style_formats = array(
		array(
			'title' => 'Lead paragraph',
			'block' => 'p',
			'classes' => 'lead'
		),
        array(
            'title' => 'Button (primary)',
            'selector' => 'a',
            'attributes' => array('class' => 'btn btn--primary btn--icon')
        ),
        array(
            'title' => 'Button (secondary)',
            'selector' => 'a',
            'attributes' => array('class' => 'btn btn--secondary btn--icon')
        ),
        array(
            'title' => 'Button (accent)',
            'selector' => 'a',
            'attributes' => array('class' => 'btn btn--accent btn--icon')
        )
	);

	$init_array['style_formats'] = json_encode($style_formats);

	return $init_array;
}
add_filter('tiny_mce_before_init', 'wapr_add_mce_styles');