get stuck with Regualer Expression

I am not good at Regular Expression,. I am trying the following:
Change the line1 to line2:
Line1: "<a href='sub1/mypage.php'>MyPage</a>"
line2: "<a href='http://mydomain.com/sub1/mypage.php'>MyPage</a>"

Of couse, if line2 stay the same when line 1 is:
"<a href='http://something.com/sub1/mypage.php'>MyPage</a>"

The point is: line2 shouldn't change if there is http:// or ftp:// or anything else!
Please help if you have clue!

 

if (strpos("$url", "://") !== false)
{
   // the url contains a ://
}
else
{
   str_replace("sub1","http://yourdomain.com/sub1",$url);
}

PHP:

Maybe start with something like this.

 

Thanks, however, maybe I still have to use eregi, because, "sub1" may be any name.

 

Further more, my problem relates to redirect URL, for example, with absolute URL:

$url="http://bh2.host.sk/"; // Or just any ABSOLUTE URL
$redirect="http://mydomain.com/mycript.php";
$link1="This is the link <a href=\"http://bh2.host.sk/sub1/PHP.doc\">The document </a>";
echo "\n<br>1.1 ".eregi_replace("href=\"([a-z]+://.+)\"","href=\"$redirect?url=\\1\"",$link1);

PHP:

But I meet problem with RELATIVE URL, for example if link2

$link1="This is the link <a href=\"sub1/PHP.doc\">The document </a>";
$domain="http://bh2.host.sk"

PHP:

I need to have a result something like:

http://mydomain.com/myscript.php?url=http://bh2.host.sk/sub1/PHP.doc

Code (markup):