Dynamically add id to heading tags

I had this very same dilemma, and I found this plugin: Add IDs to Header Tags, and it works fine for posts. (I’m not affiliated to the plugin in any way).

To make it work for pages too, I had to make a change in the code, which is explained on this support page: Not adding IDs

It seems that the developer is not active, and is not offering any support for the plugin, so if you find some problems, you have to fix them yourself, unfortunately. But the plugin is a fine base to start out from.

In this following code I removed the “is_single()” check, so that the code works on both pages and posts. Futhermore I removed some of the “extra” code to make it fit OP’s question.

//Author URI: http://stephanis.info

add_filter( 'the_content', 'add_ids_to_header_tags' );
function add_ids_to_header_tags( $content ) {

    $pattern = '#(?P<full_tag><(?P<tag_name>h\d)(?P<tag_extra>[^>]*)>(?P<tag_contents>[^<]*)</h\d>)#i';
    if ( preg_match_all( $pattern, $content, $matches, PREG_SET_ORDER ) ) {
        $find = array();
        $replace = array();
        foreach( $matches as $match ) {
            if ( strlen( $match['tag_extra'] ) && false !== stripos( $match['tag_extra'], 'id=' ) ) {
                continue;
            }
            $find[]    = $match['full_tag'];
            $id        = sanitize_title( $match['tag_contents'] );
            $id_attr   = sprintf( ' id="%s"', $id );
            $replace[] = sprintf( '<%1$s%2$s%3$s>%4$s</%1$s>', $match['tag_name'], $match['tag_extra'], $id_attr, $match['tag_contents']);
        }
        $content = str_replace( $find, $replace, $content );
    }
    return $content;
}