match subdomain for a redirect - php

Question

I want to match a subdomain that is in the PHP variable $_SERVER['SERVER_NAME'] and then do a internal redirect. Apache or nginx rewrites is not a option because this are external rewrites that are visible to the client/user.
My regular expression is (.*(?<!^.))subdomain\.example\.com as you can see I match a subdomain in a subdomain (multi-level subdomains). The first capture group would I like to use for later.
This is my PHP code:
if(preg_match('#(.*(?<!^.))subdomain\.example\.com#', $_SERVER['SERVER_NAME'], $match1)) {
echo $match1[1] . 'anothersubdomain.example.com';
}
But this will fail if the subdomain is for example csssubdomain.example.com because this is another subdomain that I don't want to match. With the following PHP script I test the matches:
$tests = array(
'subdomain.example.com' => 'anothersubdomain.example.com',
'css.subdomain.example.com' => 'css.anothersubdomain.example.com',
'csssubdomain.example.com' => 'csssubdomain.example.com',
'tsubdomain.example.com' => 'tsubdomain.example.com',
'multi.sub.subdomain.example.com' => 'multi.sub.anothersubdomain.example.com',
'.subdomain.example.com' => '.subdomain.example.com',
);
foreach( $tests as $test => $correct_answer) {
$result = preg_replace( '#(.*(?<!^.))subdomain\.example\.com#', '$1anothersubdomain.example.com', $test);
echo 'Input: ' . $test . "\n" .
'Expected: ' . $correct_answer . "\n" .
'Actual : ' .$result . "\n";
$passorfail = (strcmp( $result, $correct_answer) === 0 ? "PASS\n\n" : "FAIL\n\n");
echo $passorfail;
}
You'd get as output:
Input: subdomain.example.com
Expected: anothersubdomain.example.com
Actual : anothersubdomain.example.com
PASS
Input: css.subdomain.example.com
Expected: css.anothersubdomain.example.com
Actual : css.anothersubdomain.example.com
PASS
Input: csssubdomain.example.com
Expected: csssubdomain.example.com
Actual : cssanothersubdomain.example.com
FAIL
Input: tsubdomain.example.com
Expected: tsubdomain.example.com
Actual : tsubdomain.example.com
PASS
Input: multi.sub.subdomain.example.com
Expected: multi.sub.anothersubdomain.example.com
Actual : multi.sub.anothersubdomain.example.com
PASS
Input: .subdomain.example.com
Expected: .subdomain.example.com
Actual : .subdomain.example.com
PASS
The strange thing is that it does match csssubdomain.example.com but not tsubdomain.example.com.
Does someone know what regular expression you can use for this case? I've tried some things with lookahead and lookbehind zero-width assertions but it didn't really work.

Answer 1

You can try this pattern:
~^((?:\w+\.)*?)subdomain\.example\.com~
if you allow this .toto.subdomain.example.com, just add \.? at the beginning:
~^((?:\.?\w+\.)*?)subdomain\.example\.com~
if you want to allow a hyphen character just add it to the character class:
~^((?:\.?[\w-]+\.)*?)subdomain\.example\.com~
and if you don't allow substring to begin or ending with a hypen character:
~^((?:\.?\w+([\w-]*?\w)?\.)*?)subdomain\.example\.com~