PHP：带有模式数组的preg_replace [关闭]

I would like to replace the value of my variable $os with the preg_replace function, where I have an array of multiple regex patterns to be searched from and and array of multiple replacements to replace with. If $pattern_os[0] matches $os, it shall also be replaced with the corresponding $replace_os[0].

Here is my code:

<?php
    $useragent = $_SERVER['HTTP_USER_AGENT'];
    $pattern_os = array('/.*Mac.*/','/.*Windows.*/');
    $replace_os = array('Mac','Windows');
    $os = $useragent;
    $no_os = "No operating system was found. <br />";

    for($i=0;$i<2;$i++){
        preg_replace($pattern_os[$i], $replace_os[$i], $os);
    }

    echo $os;
?>

This is the output I get:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_2) AppleWebKit/537.75.14 (KHTML, like Gecko) Version/7.0.3 Safari/537.75.14

Of course I'd like to have an output like this:

Mac

It'd be nice if someone could help me with this problem.

I'm new here, frustrated after failing so many times and this is my first question, so telling me what to improve on asking or where to search (although I think I spent enough time googling) might also help a bit.

Thanks

You're not assigning the result of the preg_replace to a variable. You keep inputting, and not changing, $os.

This should work:

for ( $i=0; $i<2; $i++ ) {
    $os = preg_replace($pattern_os[$i], $replace_os[$i], $os);
}

and of course you could use foreach:

foreach ( $pattern_os as $i => $pattern ) {
    $os = preg_replace($pattern, $replace_os[$i], $os);
}

or at least count instead of 2:

for ( $i=0; $i<count($pattern_os); $i++ ) {