[PHP] validating input

[blackwater dev]

I want to check a string for only numbers and letters but am banging
my head with regex:

$good = "joh_'";

// Let's check the good e-mail
if (preg_match("/[a-z0-9]/", $good)) {
        echo "Good";
} else {
        echo "Bad";
}

This returns Good, why?

Thanks!

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