Add Class to first Paragraph in WordPress the_content();

functions.php

/**
 * Add Class to first Paragraph in WordPress the_content();
 *
 * @param [type] $content
 * @return void
 */
function first_paragraph($content)
{
	if (is_page('careers')) {
		return preg_replace('/<p([^>]+)?>/', '<p$1 class="text-center">', $content, 1);
	} else {
		return preg_replace('/<p([^>]+)?>/', '<p$1>', $content, 1);
	}
}
add_filter('the_content', 'first_paragraph');