PHP正则表达式只接受字母和数字，只接受一个点

Question

I don't know much about regex and I'm trying to create a username string accepts only English letters and numbers and only one dot.

I have this regex code:

if(preg_match('/[^A-Za-z0-9]/', $username)){
    $error = "Invalid Username";
}

this code allowed me to have only English letters and number, I want to add dots to this code, and accept only one dot.

Answer 1

Only one dot, but anywhere in the string? I'd make that a second check:

if(preg_match('/[^A-Za-z0-9\.]/', $username) || substr_count($username, '.') > 1){
    $error = "Invalid Username";
}