Q:

PHP string manipulation - insert values after specific piece

I have a string val of H: 35" V: 24 1/2"
I can extract the 35 and 24 perform a metric conversion calculation then, (and this is where i'm stuck),  remake the string to look like H: 35" (89 cm) V: 24 1/2" (62 cm)?
I found regex that could pull the int by
$str = 'H: 35" V: 24 1/2"';
preg_match_all('!\d+!', $str, $matches);
print_r($matches, true);  
//Array ( [0] => 35 [1] => 24 [2] => 1 [3] => 2 ) )

But after this, how can I remake the original string and insert the array values after each of the pieces?
FINAL
thank you @AbraCadaver for the answer, however I found the fraction 24 1/2 was not getting calculated so here was my final
$str = preg_replace_callback('!(\d+( \d+/\d+)?")!',
   function($matches) {

        if(isset($matches[2])){
            $fraction = $matches[2];
            if( strpos( $fraction, '/' ) !== false ) {
                $ints = explode('/', $fraction);
                $fraction = $ints[0] / $ints[1];
            }
        }

      $new  = isset($matches[2]) ? $matches[1] + $fraction : $matches[1];
        $new *= 2.54;
        return $matches[1] . " (" . round($new) . " cm)" . "<br>";
   }
   , $str);

A:

I would use preg_replace_callback(). Just add round() if needed.
Capture digits \d+ followed by an optional space followed by optional digits / digits ( \d+/\d+)? and the inch symbol ":
$str = preg_replace_callback('!(\d+( \d+/\d+)?")!',
         function($matches) {
             $new  = isset($matches[2]) ? $matches[1] + $matches[2] : $matches[1];
             $new *= 2.54;
             return $matches[1] . " ($new cm)"; //or round($new)
         }
         , $str);

