Filter the_content to add something before each <h1> element?

Question

I'm looking for a clean way of filtering the_content to add a piece of code before each H1 tag present in it. In fact, I have a long privacy policy text and I want to add a back to the top link before each paragraph. Of course, I could easily add these in the content editor but I don't want my customer to be able to remove it accidentally.

So I have something like :

<h1>BLA BLA BLA</h1>
<p>A very interesting piece of text here...<p>
<h1>BLA BLA BLA</h1>
<p>A very interesting piece of text here...<p>
<h1>BLA BLA BLA</h1>
<p>A very interesting piece of text here...<p>

and I want to filter it to get :

<a href="#">Back to the top</a>
<h1>BLA BLA BLA</h1>
<p>A very interesting piece of text here...<p>
<a href="#">Back to the top</a>
<h1>BLA BLA BLA</h1>
<p>A very interesting piece of text here...<p>
<a href="#">Back to the top</a>
<h1>BLA BLA BLA</h1>
<p>A very interesting piece of text here...<p>

Thanks in advance for your time.

Cheers,

J.

Answer 1

Just filter 'the_content', test if you are on the correct page, then run str_ireplace():

add_filter( 'the_content', 'wpse_76808_filter_h1' );

function wpse_76808_filter_h1( $content )
{
    // page slug
    if ( ! is_page( 'privacy-policy' ) )
        return $content;

    return str_ireplace( '<h1', '<a href="#">Back to the top</a><h1', $content );
}