pre_replace not sure what the correct expression is

[eluser]bugboy[/eluser]
yeah

i want to replace $tag with a a link to $title_tag

Code:

$string = preg_replace($patterns, $replacements, $string, 1);

This would the replace the the first instance of that word with a link to a page about that word.

(a really simple example to show what i mean)

Code:

$tag = 'hello';
$title_tag = "hello_page";

$patterns[] = '/'.$tag.'(?![^<]*>)/i';
$replacements[] = '<a  href="'/'.$title_tag.'">'.$title_tag.'</a>';

The thing is if there is a link already in the copy that has hello in it the i'd get nested links