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..ea92cc3 100755
--- a/src/Ulid.php
+++ b/src/Ulid.php
@@ -16,13 +16,17 @@ 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;
         }
+
+        if (!preg_match(UlidConstants::REGEX, $ulid)) {
+            return false;
+        }
         return true;
     }
 
@@ -155,9 +159,9 @@ public function generate(
         $encodingChars = UlidConstants::CHARS;
 
         for ($i = 9; $i >= 0; $i--) {
-            $mod = $time % UlidConstants::LENGHT;
+            $mod = $time % UlidConstants::LENGTH;
             $timeChars = $encodingChars[$mod] . $timeChars;
-            $time = ($time - $mod) / UlidConstants::LENGHT;
+            $time = ($time - $mod) / UlidConstants::LENGTH;
         }
 
         $this->hasIncrementLastRandChars(
@@ -195,7 +199,7 @@ public function decodeTime(
             if ($encodingIndex === false) {
                 throw new Exception('Invalid ULID character: ' . $char);
             }
-            $exponential = pow(UlidConstants::LENGHT, $index);
+            $exponential = pow(UlidConstants::LENGTH, $index);
             $carry += ($encodingIndex * $exponential);
         }
 
diff --git a/src/UlidConstants.php b/src/UlidConstants.php
index 992a9c2..d0936d6 100755
--- a/src/UlidConstants.php
+++ b/src/UlidConstants.php
@@ -5,8 +5,9 @@
 class UlidConstants
 {
     const CHARS = '0123456789ABCDEFGHJKMNPQRSTVWXYZ';
-    const LENGHT = 32;
+    const LENGTH = 32;
     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..6351b95 100755
--- a/tests/UlidTest.php
+++ b/tests/UlidTest.php
@@ -230,6 +230,32 @@ public function testDecodeTimeInvalidTime()
         $ulid->decodeTime('QS234SAD23RADSWRA3FADQ3RS2');
     }
 
+    /**
+ * @covers Ulid\Ulid::isValidFormat
+ */
+    public function testIsInvalidCharactersFormat()
+    {
+        $ulid = new Ulid();
+        
+        $invalidUlid = '%%%%%%%%%%%%%%%%%%%%%%%%%%';
+
+        $result = $ulid->isValidFormat($invalidUlid);
+
+        $this->assertFalse($result);
+    }
+
+        /**
+     * @covers Ulid\Ulid::isValidFormat
+     */
+    public function testIsValidFormatDoesNotValidateCharacters()
+    {
+        $ulid = new Ulid();
+
+        $result = $ulid->isValidFormat('01E475VQGHNW990PVHXFDT4C6W');
+
+        $this->assertTrue($result);
+    }
+
     protected function tearDown(): void
     {
         //