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<?php

// Add class to links generated by next_posts_link and previous_posts_link,
// see https://css-tricks.com/snippets/wordpress/add-class-to-links-generated-by-next_posts_link-and-previous_posts_link/
add_filter('next_posts_link_attributes', 'posts_link_attributes');
add_filter('previous_posts_link_attributes', 'posts_link_attributes');

function posts_link_attributes() {
    return 'class="button"';
}

// Add class to links generated by next_post_link and previous_post_link,
// see http://wordpress.stackexchange.com/questions/17218/how-to-add-css-class-to-previous-post-link-or-get-previous-next-post-link-url
function posts_link_next_class($format){
     $format = str_replace('href=', 'class="button" href=', $format);
     return $format;
}
add_filter('next_post_link', 'posts_link_next_class');

function posts_link_prev_class($format) {
     $format = str_replace('href=', 'class="button" href=', $format);
     return $format;
}
add_filter('previous_post_link', 'posts_link_prev_class');

?>