clickable url's & emails

I need to make url's & email addresses clickable but to not mess with already formatted links & emails.

I am using this now but it messes up already formatted links:

$Text=ereg_replace ('[_a-zA-Z0-9\-]+(\.[_a-zA-Z0-9\-]+)*\@'
.'[_a-zA-Z0-9\-]+(\.[a-zA-Z]{1,3})', '<a
href="mailto:\\0">\\0</a>',$Text);

$Text =
ereg_replace("[[:alpha:]]+://[^<>[:space:]]+[[:alnum:]/]","<a
href=\"\\0\" target=\"_blank\">\\0</a>",
$Text);

Anybody know how to do this?