PHP RegEx to prevent repeating characters

Question

if(preg_match("%(?=.{8})[A-Za-z0-9]([.-]{0,1})%", stripslashes(trim($_POST['username']))))

I want the username to be at least 8 characters A-Za-z0-9 with only 1 . or 1 - . How do I implement this? My code is allowing a username of ".............." as valid. <- Don't want that.

BTW: the "." or "-" is meant to be optional.

Ry- · Accepted Answer · 2012-07-03 23:11:45Z

4

Sometimes, regular expressions just aren't the right tool for the job.

$username = stripslashes(trim($_POST['username']));

if(preg_match('/^[a-zA-Z0-9.-]{8,}$/', $username)
&& substr_count($username, '-') <= 1
&& substr_count($username, '.') <= 1) {
    # It's valid.
}

answered Jul 3, 2012 at 23:11

Ry-♦

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Comments

LSerni · Accepted Answer · 2012-07-03 23:32:28Z

2

A quick patch would be to count those characters after the first check.

i.e.,

 if (strlen(preg_replace("#[^.-]#", '', $username)) > 2)
     // Not valid.

This is not the check you asked for, this counts .'s and -'s together, so two dots or two dashes would be considered valid. But it's quick and easily extended to more characters.

edited Jul 3, 2012 at 23:32

answered Jul 3, 2012 at 23:14

LSerni

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8 Comments

MrPizzaFace Over a year ago

Ty for your response however minitech's solution worked for me.

LSerni Over a year ago

Good! But I was going to suggest a change -- maybe "/[A-Z][A-Za-z0-9.-]{7,}/" would do better, because I don't think you would like usernames like ".htaccess" :-).

MrPizzaFace Over a year ago

I guess I can make that change. Just curious though how would a username such as ".htaccess" be malicious? Meaning would it be a threat to my security in anyway by allowing a person to have this username? Ty for your input btw

MrPizzaFace Over a year ago

btw the correct code to prevent a username like .htaccess and still adhere to my other requirements would be: '/^[A-Za-z0-9]+[A-Za-z0-9.-]{8,}$/'

LSerni Over a year ago

No, no, no security problem whatsoever. Actually I'd love an username like that, it would be cool :-)

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