PHP Links Regex

How to modify this function so it could add target="_blank" attribute to external links? Take the default domain as example.com

function makeLinks($text){
 if(eregi_replace('(((f|ht){1}tp://)[-a-zA-Z0-9@:%_\+.~#?&//=]+)', '<a href="\\1">\\1</a>', $text) != $text){ 
  $text = eregi_replace('(((f|ht){1}tp://)[-a-zA-Z0-9@:%_\+.~#?&//=]+)', '<a href="\\1">\\1</a>', $text);
  return $text;
 } 
 $text = eregi_replace('(www\.[-a-zA-Z0-9@:%_\+.~#?&//=]+)', '<a href="http://\\1">\\1</a>', $text); // ([[:space:]()[{}]) deleted from beginnig of regex 
 $text = eregi_replace('([_\.0-9a-z-]+@([0-9a-z][0-9a-z-]+\.)+[a-z]{2,3})', '<a开发者_C百科 href="mailto:\\1">\\1</a>', $text); 
 return $text; 
}

Parsing HTML is unestable, expensive and, in a word, a hell. You should use Simple HTML DOM Parser.

<?php
    class HtmlLinkUtility
    {
        public static $BaseDomain = null;
        public static function ReplaceEmailToHtmlLink($source)
        {
            return preg_replace('/([_.0-9a-z-]+@([0-9a-z][0-9a-z-]+\.)+[a-z]{2,3})/i',
                '<a href="mailto:\1">\1</a>', $source);
        }

        public static function ReplaceUrlToHtmlLink($source)
        {
            function replaceUrl($groups) {
                $url = $groups[1];
                return '<a href="' . $url . '"' . (strpos($url, HtmlLinkUtility::$BaseDomain) !== false ?
                    ' target="_blank"' : '') . '>' . $url . '</a>';
            }

            return preg_replace_callback('!(((f|ht){1}tp://)[-a-zA-Z0-9@:%_+.~#?&//=]+)!i',
                replaceUrl, $source);
        }

        public static function ReplaceTextDataToLinks($source, $baseDomain)
        {
            self::$BaseDomain = $baseDomain;
            return self::ReplaceUrlToHtmlLink(self::ReplaceEmailToHtmlLink($source));
        }
    }

    echo HtmlLinkUtility::ReplaceTextDataToLinks(
        "test@test.com<br />http://www.google.com/<br />http://www.test.com/",
        "google.com"
    );
?>

Could not see why you would use two expressions that basically matched/replaced the same thing. Simplified your method a little.

Also, for the record. HTML is not regular in that way that it in any form could be parsed by a regular expression. Though, for simple cases like the one above, it works well.