replace all "foo" between two HTML tags using REGEX (PHP code)

Question

I want a regex code , to replace all "foo" strings to "bar" , between the html tags pre>< /pre>

here is an example :

< html>
< p> blah blah blah foo try foo< /p>
< pre> foo try foo word foofoo < /pre>
< /html>

shoud be

< html>
< p> blah blah blah foo try foo< /p>
< pre> bar try bar word barbar < /pre>
< /html>

so , that's means all foo between the tags pre should be replaced by .

i tried to use this regex pattern but its not working.

do {
$string = preg_replace('/< pre>([^)]*)foo([^)]*< /pre>)/U', '1boo2', $string, -1,$count);
}while($count != 0);
echo $string;

im sorry for my english Thank you,

Answer 1

$string = '< html>
< p> blah blah blah foo try foo< /p>
< pre> foo try foo word foofoo < /pre>
< /html>';
$string = preg_replace('/foo(?=(?:.(?!< pre>))*< /pre>)/Um', 'boo', $string);
echo $string;