preg_replace function problem

[eluser]basty_dread[/eluser]
the one with link is

Code:

<a href='yahoo.com'>Web design las vegas</a>

this is where we will search:

Code:

$string = "WEB DESIGN Web Design web design <a href='yahoo.com'>web design google</a> las vegas";

i used this preg_replace function, but it replaced web design google las vegas turning the result into:

Code:

<a href='yahoo.com'><a href="http://google.com">web design</a> google</a> las vegas