How to include line-breaks in the_excerpt?

Same to Johannes Pille's solution, the bellowing solution should be more adaptive.

In detail:

• override function wp_trim_excerpt
• remove old filter and add new custom new filter

Here's full code:

// append to themes/{your_theme}/functions.php

define('EXCERPT_RARELY', '{[}]');
define('EXCERPT_BR', nl2br(PHP_EOL));

function so306588_trim_excerpt_custom($text = '')
{
    add_filter('the_content', 'so306588_trim_excerpt_custom_mark', 6);

    // get through origin filter
    $text = wp_trim_excerpt($text);

    remove_filter('the_content', 'so306588_trim_excerpt_custom_mark', 6);

    return so306588_trim_excerpt_custom_restore($text);
}

function so306588_trim_excerpt_custom_mark($text)
{
    $text = nl2br($text);
    return str_replace(EXCERPT_BR, EXCERPT_RARELY, $text);
}

function so306588_trim_excerpt_custom_restore($text)
{
    return str_replace(EXCERPT_RARELY, EXCERPT_BR, $text);
}

// remove default filter
remove_filter('get_the_excerpt', 'wp_trim_excerpt');

// add custom filter
add_filter('get_the_excerpt', 'so306588_trim_excerpt_custom');

wouldn't it be simpler to apply the_content filter to the excerpt?

i only use this occasionally, right within the template file:

$excerpt = apply_filters('the_content',  ( get_post(get_the_ID())->post_excerpt) );
echo $excerpt;

also could just use PHP's nl2br() function:

echo nl2br(get_the_excerpt());

-- or --

nl2br(the_excerpt());

At the moment, it is possible to use the_excerpt() funtion to display the excerpt with line breaks. But if you like to return the result you can use for that purpose a function like this:

function get_the_excerpt_theme() {
    $excerpt = get_the_excerpt();
    $excerpt = apply_filters( 'the_excerpt', $excerpt );
    return $excerpt;
}

It applies a filters to the excerpt using apply_filters('the_excerpt', $excerpt ) like in the wp function the_excerpt() but returns the string without displaying it.

Also, if you like to allow only line breaks in the excerpt, you can add $excerpt = strip_tags($excerpt,'<br>') below the line apply_filters.

Hope it helps!

There is no filter that would allow you to set allowable tags not to be removed by the_excerpt(). Arguably a shortcoming of the core.

Anyhow, the actual excerpt generation does not happen in that template tag but entirely elsewhere:

Excerpts are generated by the function wp_trim_excerpt(), inside of which the excerpt filters you are already using (excerpt_length and excerpt_more) are applied and which calls wp_trim_words(), which in turn calls upon wp_strip_all_tags(). All three functions are located in wp-includes/formatting.php

Hence in the absence of a filter for the case and the inevitability of your excerpt running through wp_strip_all_tags(), the only possibility to preserve some tags is adding a custom replacement function for wp_trim_excerpt():

function wpse67498_wp_trim_excerpt( $text = '' ) {
    $raw_excerpt = $text;

    if ( '' == $text ) {
        $text = get_the_content( '' );
        $text = strip_shortcodes( $text );
        $text = apply_filters( 'the_content', $text );
        $text = str_replace( ']]>', ']]>', $text );
        $excerpt_length = apply_filters( 'excerpt_length', 55 );
        $excerpt_more = apply_filters( 'excerpt_more', ' ' . '[...]' );

        $allowable = '<br>';
        $text = preg_replace( '@<(script|style)[^>]*?>.*?</\\1>@si', '', $text );
        $text = trim( strip_tags( $text, $allowable ) );

        if ( 'characters' == _x( 'words', 'word count: words or characters?' )
            && preg_match( '/^utf\-?8$/i', get_option( 'blog_charset' ) ) )
        {
            $text = trim( preg_replace( "/[\n\r\t ]+/", ' ', $text ), ' ' );
            preg_match_all( '/./u', $text, $words_array );
            $words_array = array_slice( $words_array[0], 0, $num_words + 1 );
            $sep = '';
        } else {
            $words_array = preg_split( "/[\n\r\t ]+/", $text, $num_words + 1, PREG_SPLIT_NO_EMPTY );
            $sep = ' ';
        }

        if ( count( $words_array ) > $excerpt_length ) {
            array_pop( $words_array );
            $text = implode( $sep, $words_array );
            $text = $text . $excerpt_more;
        } else {
            $text = implode( $sep, $words_array );
        }
    }

    return apply_filters( 'wp_trim_excerpt', $text, $raw_excerpt );
}

remove_filter( 'get_the_excerpt', 'wp_trim_excerpt');
add_filter( 'get_the_excerpt', 'wpse67498_wp_trim_excerpt' );