diff --git a/docker-compose.yml b/docker-compose.yml
index 03d83cb..aa16942 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -2,7 +2,7 @@ services:
   ulid-php-lib:
     build:
       context: ./
-      dockerfile: ./ops/Dockerfile
+      dockerfile: ./contrib/Dockerfile
     container_name: ulid-php-lib
     volumes:
       - ./:/var/www/html
diff --git a/src/Ulid.php b/src/Ulid.php
index d0d4bef..722f008 100755
--- a/src/Ulid.php
+++ b/src/Ulid.php
@@ -16,14 +16,18 @@ class Ulid
      * @param string $ulid
      * @return bool
      */
-    public function isValidFormat(
-        string $ulid
-    ): bool {
+    public function isValidFormat(string $ulid): bool
+    {
         $validator = UlidConstants::TIME_LENGTH + UlidConstants::RANDOM_LENGTH;
+
         if (strlen($ulid) !== $validator) {
             return false;
         }
-        return true;
+
+        return preg_match(
+            UlidConstants::REGEX,
+            $ulid
+        ) === 1;
     }
 
     /**
diff --git a/src/UlidConstants.php b/src/UlidConstants.php
index 992a9c2..58dcc11 100755
--- a/src/UlidConstants.php
+++ b/src/UlidConstants.php
@@ -9,4 +9,5 @@ class UlidConstants
     const TIME_MAX = 281474976710655;
     const TIME_LENGTH = 10;
     const RANDOM_LENGTH = 16;
+    const REGEX = '/^[0-9A-HJKMNP-TV-Z]{26}$/ ';
 }
diff --git a/tests/UlidTest.php b/tests/UlidTest.php
index 5689cdf..9e6ad7b 100755
--- a/tests/UlidTest.php
+++ b/tests/UlidTest.php
@@ -230,6 +230,19 @@ public function testDecodeTimeInvalidTime()
         $ulid->decodeTime('QS234SAD23RADSWRA3FADQ3RS2');
     }
 
+      /**
+     * @covers Ulid\Ulid::isValidFormat
+     */
+    public function testIsValidFormatDoesNotValidateCharacters()
+    {
+        $ulid = new Ulid();
+
+        $result = $ulid->isValidFormat('01E475VQGHNW990PVHXFDT4C6W');
+
+        $this->assertTrue($result);
+    }
+
+
     protected function tearDown(): void
     {
         //