WordPress: convert bbcode in ACF field messages and instructions to HTML markup.

function.php

/**
 * Filter: convert bbcode into html markup for added emphasis to ACF fields.
 *
 * @note thanks to StackOverflow https://stackoverflow.com/questions/24200048/show-bbcode-in-html-page-with-php
 * @return $field
 */
add_filter('acf/prepare_field', function($field){

    // Are both viable fields empty; else: bail out.
    if (empty($field['message']) && empty($field['instructions']))
        return $field;

    // Target patterns
    $search = array (
        '/(.*?)(\[br\])(.*?)/',
        '/(\[b\])(.*?)(\[\/b\])/',
        '/(\[i\])(.*?)(\[\/i\])/',
        '/(\[u\])(.*?)(\[\/u\])/',
        '/(\[ul\])(.*?)(\[\/ul\])/',
        '/(\[li\])(.*?)(\[\/li\])/',
        '/(\[url=)(.*?)(\])(.*?)(\[\/url\])/',
        '/(\[url\])(.*?)(\[\/url\])/'
    );

    // Replacement markup
    $replace = array (
        '$1<br/>$3',
        '<strong>$2</strong>',
        '<em>$2</em>',
        '<u>$2</u>',
        '<ul>$2</ul>',
        '<li>$2</li>',
        '<a href="$2" target="_blank">$4</a>',
        '<a href="$2" target="_blank">$2</a>'
    );

    // Replace message
    if (!empty($field['message']))
        $field['message'] = preg_replace($search, $replace, $field['message']);

    // Replace instructions
    if (!empty($field['instructions']))
        $field['instructions'] = preg_replace($search, $replace, $field['instructions']);

    // Return $field
    return $field;
});