Username may contain lowercase characters and numbers - php

Question

I want to allow lowercase characters and numbers in username field.
But with following conditions...
Only numbers as username NOT allowed (e.g. only mobile number)
Only lowercase characters allowed (e.g. without any number in username)
Lowercase characters + numbers allowed (e.g. combination of lowercase and numbers)
Minimum length 8 characters required
Maximum length 20 characters allowed
What php regex will do it ?
I tried with following, but it forces lowercase + numbers. Only lowercase username not allowing.
$username_pattern = '/^(?=.*[a-z])(?=.*[a-z])(?=.*\d)[a-z0-9]{8,20}$/';
I want only lowercase and/or lowercase+numbers ( min 8 and max 20 ) in username
Help appreciated.

Answer 1

You can simplify it to not allowing only digits
^(?!\d*$)[a-z0-9]{8,20}$
Explanation
^ Start of string
(?!\d*$) Negative lookahead, assert not only digits till end of string
[a-z0-9]{8,20} Match 8-20 times a char a-z or a digit 0-9
$ End of string
Regex demo | Php demo
$username_pattern = '/^(?!\d*$)[a-z0-9]{8,20}$/';
$userNames = [
"1a3b5678",
"1a3b5678abcd",
"12345678",
"1a3b5678abcddddddddddddddddddddddddddddddd",
"1a3B5678",
"a1"
];
foreach ($userNames as $userName) {
if (preg_match($username_pattern, $userName)) {
echo "Match - $userName" . PHP_EOL;
} else {
echo "No match - $userName" . PHP_EOL;
}
}
Output
Match - 1a3b5678
Match - 1a3b5678abcd
No match - 12345678
No match - 1a3b5678abcddddddddddddddddddddddddddddddd
No match - 1a3B5678
No match - a1