Limit preg_replace to two total replacements

Question

I have the following PHP code that makes HTML code for images from a predefined list:

$message = "thing.png, other.jpg, other.jpg, other.jpg, last.tiff, thing.png";

$keywords = array("thing.png", "other.jpg");
foreach ($keywords as &$value) $value = "/(".$value.")/";
echo $message = preg_replace($keywords, '<img src="images/$1" />', $message, 2);

I added a 2 to limit preg_replace to make a maximum of two replacements.

This would generate following output:

<img src="images/thing.png">, <img src="images/other.jpg">, <img src="images/other.jpg">, other.jpg, last.tiff, <img src="images/thing.png">

I would like this instead:

<img src="images/thing.png">, <img src="images/other.jpg">, other.jpg, other.jpg, last.tiff, thing.png

Note that my desired behaviour limits preg_replace to two replacements total, not two replacements per array (or needle).

Could you help me limit the code to two total replacements? Is this what preg_replace_callback is for?

---

My final code:

$message = "thing.png, other.jpg, other.jpg, other.jpg, last.tiff, thing.png";

$keywords = array("thing.png", "other.jpg");
$regexp = "/".implode("|", array_map("preg_quote", $keywords))."/";
echo $message = preg_replace($regexp, '<img src="images/$0" />', $message, 2);

Answer 1

preg_replace takes a reference parameter that it fills in with the number of replacements made. You can use this to subtract from the total, until you reach your limit.

$total = 2;
foreach ($keywords as $value) {
    $regexp = "/" . preg_quote($value) . "/";
    $message = preg_replace($regexp, '<img src="$image/$0" />', $message, $total, $count);
    $total -= $count;
    if ($total == 0) {
        break;
    }
}

Or you could combine them all into a single regexp using alternation:

$regexp = implode('|', array_map('preg_quote', $keywords));
$message = preg_replace("/$regexp/", '<img src="$image/$0" />', $message, 2);