Limit the_excerpt with max of x characters

Question

What I'm trying to do:
I want to display the_excerpt, but I have a maximum of x characters the_excerpt may use, but I don't want to display a couple of characters of a word, only whole words.

Some more information:
This code is on a non-single/non-permalink web page such as archives, categories, front page, and searches, that makes me unable to use <!--more-->.

The code:
This is the code I use:

add_filter( 'excerpt_length', function( ) {
    return 20;
} );

if ( have_posts() ):
    while( have_posts() ): the_post();
        the_excerpt( );
    endwhile;
endif;

Answer 1

add_filter('wp_trim_excerpt', function($text){    
   $max_length = 140;

   if(mb_strlen($text, 'UTF-8') > $max_length){
     $split_pos = mb_strpos(wordwrap($text, $max_length), "\n", 0, 'UTF-8');
     $text = mb_substr($text, 0, $split_pos, 'UTF-8');
   }

   return $text;
});

This should take into account your max length and split the text at the nearest word boundary. Apply the filter, and call the_excerpt(); in your templates

---

Apparently there's a wp_trim_words function from WP 3.3 that you can also use, but from the source looks very inefficient. Appart from using 3 regexes, it splits the text into an array of words, and this can get very slow and memory exhaustive for large chunks of text...

Answer 2

After some puzzling, I found this solution:

$limit_characters = 120;

function new_excerpt_length( ) {
    global $length;
    return $length;
}

function get_the_excerpt_special( ) {
    global $limit_characters, $length;

    if(strlen( get_the_excerpt() ) > $limit_characters ) {
        $length--;
        return get_the_excerpt_special();
    } else {
        the_excerpt();
    }
}

if ( have_posts() ):
    while( have_posts() ): the_post();
        add_filter( 'excerpt_length', 'new_excerpt_length' );

        $length = 20;
        get_the_excerpt_special( );

        remove_filter( 'excerpt_length', 'new_excerpt_length' );

    endwhile;
endif;

It works! YAY! But since it has to repeatably call get_the_excerpt, the page slows down a bit.