diff --git a/src/Validator.php b/src/Validator.php
index e70afd3..2499849 100644
--- a/src/Validator.php
+++ b/src/Validator.php
@@ -41,6 +41,8 @@ class Validator
         'SE' => '\d{12}',
         'SI' => '\d{8}',
         'SK' => '\d{10}',
+        'SM' => '\d{5}',
+        'XI' => '(\d{3} ?\d{4} ?\d{2}( ?\d{3})?|[A-Z\d]{5})',
     ];
 
     /**
diff --git a/tests/ValidatorTest.php b/tests/ValidatorTest.php
index ce528c2..bd0c23a 100644
--- a/tests/ValidatorTest.php
+++ b/tests/ValidatorTest.php
@@ -53,6 +53,11 @@ public function testValidateVatNumberFormat(): void
             'SE123456789012',
             'SI12345678',
             'SK1234567890',
+            'XI123456789',
+            'XI123 4567 89',
+            'XI123456789111',
+            'XI123 4567 89 111',
+            'XIHZ6AB',
         ];
 
         $validator = new Validator();
@@ -94,6 +99,7 @@ public function testValidateVatNumberFormat(): void
             'SE12345678901',
             'SI1234567',
             'SK123456789',
+            'XI12345678',
 
             // valid number but with prefix
             'invalid_prefix_IE1234567X',
@@ -101,6 +107,7 @@ public function testValidateVatNumberFormat(): void
             'invalid_prefix_BE0123456789',
             'invalid_prefix_MT12345678',
             'invalid_prefix_LT123456789',
+            'invalid_prefix_XI123456789',
 
             // valid number but with suffix
             'IE1234567X_invalid_suffix',
@@ -108,6 +115,7 @@ public function testValidateVatNumberFormat(): void
             'BE0123456789_invalid_suffix',
             'MT12345678_invalid_suffix',
             'LT123456789_invalid_suffix',
+            'XI123456789_invalid_suffix',
         ];
 
         foreach ($invalid as $format) {