Should you initialize $matches before calling preg_match?

Question

preg_match accepts a $matches argument as a reference. All the examples I've seen do not initialize it before it's passed as an argument. Like this:

preg_match($somePattern, $someSubject, $matches);
print_r($matches);

Isn't this error-prone? What if $matches already contains a value? I would think it should be initialized to an empty array before passing it in as an arg. Like this:

$matches = array();
preg_match($somePattern, $someSubject, $matches);
print_r($matches);

Am i just being paranoid?

Answer 1

There is no need to initialise $matches as it will be updated with the results. It is in effect a second return value from the function.

Answer 2

as Chris Lear said you don't need to initialize $matches. But if your pattern contains a capture group you want to use later, it is better to write:

$somePattern = '/123(456)/';
if (preg_match($somePattern, $someSubject, $matches)) {
    print_r($matches[1]);
}

to avoid the error of undefined index in the result array. However, you can use isset($matches[1]) to check if the index exists.