target-blank.php

<?php
/*
Plugin Name: Add target="_blank" to external links
Description: This will filter links in posts and pages and add target="_blank" to those links
Version: 1.0
Author: Jan Dembowski
Author URI: http://blog.dembowski.net/
*/

add_filter( 'the_content' , 'mh_add_blank' );
// add_filter( 'comment_text' , 'mh_add_blank' );

function mh_add_blank( $content ) {

// Regex to put all <a href="http://some-url-here/path/etc" into an array
$mh_url_regex = "/\<a\ href\=\"(http|https)\:\/\/[a-zA-Z0-9\-\.]+[a-zA-Z]{2,3}.*\"[\ >]/";

preg_match_all( $mh_url_regex , $content, $mh_matches );

// Go through that array and add target="_blank" to external links
for ( $mh_count = 0; $mh_count < count( $mh_matches[0] ); $mh_count++ )
        {
        $mh_old_url = $mh_matches[0][$mh_count];
        // $mh_new_url = str_replace( '" ' , '" target="_blank" ' , $mh_matches[0][$mh_count] );
        $mh_new_url = str_replace( '">' , '" target="_blank">' , $mh_matches[0][$mh_count] );

        // Array of destinations we don't want to apply the hack to.
        // Your home URL will get excluded but you can add to this array.
        // Partial matches work here, the more specific the better.

        $mh_ignore = array(
                home_url( '/' ),
                'wordpress.org/'
                );

        // Make the substitution on all links except the ignore list
        if( !mh_array_find( $mh_old_url , $mh_ignore ) )
                $content = str_replace( $mh_old_url  , $mh_new_url , $content );
        }

return $content;
}

// Only see if the array element is contained in the string
function mh_array_find( $needle , $haystack ) {
        if(!is_array($haystack)) return false;
        foreach ($haystack as $key=>$item) {
                // See if the item is in the needle
                if (strpos($needle, $item ) !== false) return true;
        }
        return false;
}