Fixing Hostname validator subdomain underscore allowed

Author: bacinsky

src/Hostname.php

@@ -2090,6 +2090,7 @@ public function isValid($value)
 
                     // Check each hostname part
                     $check = 0;
+                    $lastDomainPart = end($domainParts);
                     foreach ($domainParts as $domainPart) {
                         // Decode Punycode domain names to IDN
                         if (strpos($domainPart, 'xn--') === 0) {
@@ -2118,7 +2119,9 @@ public function isValid($value)
 
                         // Check each domain part
                         $checked = false;
-                        foreach ($regexChars as $regexKey => $regexChar) {
+                        $isSubDomain = $domainPart != $lastDomainPart;
+                        $partRegexChars = $isSubDomain ? ['/^[a-z0-9_\x2d]{1,63}$/i'] + $regexChars : $regexChars;
+                        foreach ($partRegexChars as $regexKey => $regexChar) {
                             $status = preg_match($regexChar, $domainPart);
                             if ($status > 0) {
                                 $length = 63;

test/HostnameTest.php

@@ -119,6 +119,30 @@ public function testDashes()
         }
     }
 
+    /**
+     * Ensure the underscore character tests work as expected
+     *
+     */
+    public function testUnderscores()
+    {
+        $valuesExpected = [
+            [Hostname::ALLOW_DNS, true, [
+                '_subdomain.domain.com', 'subdomain_.domain.com', 'sub_domain.domain.com', 'sub__domain.domain.com'
+            ]],
+            [Hostname::ALLOW_DNS, false, ['_domain.com', 'domain_.com', 'do_main.com']]
+        ];
+        foreach ($valuesExpected as $element) {
+            $validator = new Hostname($element[0]);
+            foreach ($element[2] as $input) {
+                $this->assertEquals(
+                    $element[1],
+                    $validator->isValid($input),
+                    implode("\n", $validator->getMessages()) . $input
+                );
+            }
+        }
+    }
+
     /**
      * Ensures that getMessages() returns expected default value
      *