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How do auto populate the excerpt field with the first 15 words of the post content? I found this snippet:

function wps_excerpt_content( $content ) {
    $content = "YOUR EXCERPT HERE";
    return $content;
}
add_filter( 'default_excerpt', 'wps_excerpt_content' );

But I don't know how to modify it with what I need. Any help?

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2 Answers 2

up vote 3 down vote accepted

How about using the excerpt_length filter?

function my_excerpt_lenght( $length ) {
    return 15;
}
add_filter( 'excerpt_length', 'my_excerpt_lenght', 999 );
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The OP code snippet is for setting the default Excerpt when creating a post: http://example.com/wp-admin/post-new.php. So, it's not useful for the desired action.
Also, check the final note for a more detailed use of that filter.


At first, I thought of the save_post action to do it, but found a nice tip in this answer from Eugene Manuilov.

The following code auto-populates the post_excerpt whenever a post is created or saved, if it had no previous excerpt.
Note that the excerpt creation can be greatly improved, as it doesn't differentiates Urls nor short words.

// Define the custom excerpt length
$wpse_40574_custom_excerpt_length = 15;

add_filter( 'wp_insert_post_data', 'wpse_40574_populate_excerpt', 99, 2 );

/** 
 * Checks if the the post has excerpt or not
 * Code reference: http://wordpress.stackexchange.com/a/52897/12615
 */
// 
function wpse_40574_populate_excerpt( $data, $postarr ) 
{   
    global $wpse_40574_custom_excerpt_length;

    // check if it's a valid call
    if ( !in_array( $data['post_status'], array( 'draft', 'pending', 'auto-draft' ) ) && 'post' == $data['post_type'] ) 
    {
        // if the except is empty, call the excerpt creation function
        if ( strlen($data['post_excerpt']) == 0 ) 
            $data['post_excerpt'] = wpse_40574_create_excerpt( $data['post_content'], $wpse_40574_custom_excerpt_length );
    }

    return $data;
}

/** 
 * Returns the original content string if its word count is lesser than $length, 
 * or a trimed version with the desired length.
 * Reference: see this StackOverflow Q&A - http://stackoverflow.com/q/11521456/1287812
 */
function wpse_40574_create_excerpt( $content, $length = 20 )
{
    $the_string = preg_replace( '#\s+#', ' ', $content );
    $words = explode( ' ', $the_string );

    /**
     * The following is a more efficient way to split the $content into an array of words
     * but has the caveat of spliting Url's into words ( removes the /, :, ., charachters )
     * so, not very useful in this context, could be improved though.
     * Note that $words[0] has to be used as the array to be dealt with (count, array_slice)
     */
    //preg_match_all( '/\b[\w\d-]+\b/', $content, $words );

    if( count($words) <= $length ) 
        $result = $content;
    else
        $result = implode( ' ', array_slice( $words, 0, $length ) );

    return $result;
}

Note

Example of default_excerpt filter usage:

add_filter( 'default_excerpt', 'wpse_40574_excerpt_content', 10, 2 );

function wpse_40574_excerpt_content(  $post_excerpt, $post  ) 
{
    // Do nothing if not the correct post type
    // http://codex.wordpress.org/Post_Types
    if( 'post' != $post->post_type )
        return;

    $post_excerpt = "DEFAULT EXCERPT FOR POSTS OF THE TYPE -POST-";

    return $post_excerpt;
}
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