Validate that string contains only numbers, letters, and underscores

Question

How do you check if a string contains invalid characters?

I want to restrict each user's username with PHP to having numbers, letters, and underscores.

Answer 1

You can use a regular expression:

if (preg_match("^[0-9A-Za-z_]+$", username) == 0) {
    echo "<p>Invalid username</p>";
}