the right regex for a subdomain

I have a webapp, where people signup and get a sub domain und开发者_运维知识库er this app domain ( xx.app.com ) ... for each subdomain there is a db that is attached to it grammatically and have the same name as the subdomain.

what i need is the right regex that works with the subdomain and off course a db name ( mysql if it matters ), it's supposed to be lowercase & the length between 6 and 20 and the only allowed character is the " - ", also numbers are banned ...

i tried many times but it always go bad, .. some like : /([a-z-]){6,20}/

Thanks in advance :)

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There might be a right regex for this, but regex isn't right for this.

Try parse_url

Edit:

I am not sure how you are using it. If you are only processing the subdomain part, the following should work and not match numbers:

^[a-z-]{6,20}$

This ensures that the subdomain has only a to z and - and between 6 and 20 times. The ^ matches the beginning of the string and $ matches the end.

The reason the earlier regex was accepting numbers or anything else too was because the match itself would have been a part of the string. Now with the ^ and $ you are ensuring that it is the entire string.

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This would be a safer regex, since a subdomain cannot start with an hyphen:

^[a-z][a-z-]{5,19}$

As for the database name I believe it cannot contain an hyphen since it is the subtraction operator, so your best choice might be to either disallow hypens or replace them with underscores:

$database = str_replace('-', '_', $subdomain);

EDIT: Apparently @nikic is right, you can use hyphens as long as you backtick the database name.

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Have you tried escaping the hyphen?

/([a-z\-]){6,20}/

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You will need positive lookahead regex for this. Try following code:

<?php
   $a = array("xx-yyy.domain.cam", "xx4yyy.domain.cam", "abcde.domain.com", "my-sub-domain.domain.org");
   foreach ($a as $v) {
      echo "For domain $v: ";
      preg_match('/^(?:[-a-z]{6,20})(?=\.)/', $v, $m );
      if (count($m) > 0)
         echo( "subdomain: " . $m[0] . "\n");
      else
         echo "subdomain not matched\n";
   }
?>

Basically match combination of lower case alphabets and hyphen - character of 6 to 20 character length before appearance of first dot . character.

- hyphen need not be escaped if used at the start in square brackets.

OUTPUT

For domain xx-yyy.domain.cam: subdomain: xx-yyy
For domain xx4yyy.domain.cam: subdomain not matched
For domain abcde.domain.com: subdomain not matched
For domain my-sub-domain.domain.org: subdomain: my-sub-domain