diff --git a/src/Validator.php b/src/Validator.php
index 2e811dc..3dd5aa0 100644
--- a/src/Validator.php
+++ b/src/Validator.php
@@ -43,6 +43,7 @@ class Validator
         '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 64d78f8..320f856 100644
--- a/tests/ValidatorTest.php
+++ b/tests/ValidatorTest.php
@@ -54,6 +54,11 @@ public function testValidateVatNumberFormat(): void
             'SE123456789012',
             'SI12345678',
             'SK1234567890',
+            'XI123456789',
+            'XI123 4567 89',
+            'XI123456789111',
+            'XI123 4567 89 111',
+            'XIHZ6AB',
         ];
 
         $validator = new Validator();
@@ -96,6 +101,7 @@ public function testValidateVatNumberFormat(): void
             'SE12345678901',
             'SI1234567',
             'SK123456789',
+            'XI12345678',
 
             // valid number but with prefix
             'invalid_prefix_GB999999973',
@@ -104,6 +110,7 @@ public function testValidateVatNumberFormat(): void
             'invalid_prefix_BE0123456789',
             'invalid_prefix_MT12345678',
             'invalid_prefix_LT123456789',
+            'invalid_prefix_XI123456789',
 
             // valid number but with suffix
             'IE1234567X_invalid_suffix',
@@ -111,6 +118,7 @@ public function testValidateVatNumberFormat(): void
             'BE0123456789_invalid_suffix',
             'MT12345678_invalid_suffix',
             'LT123456789_invalid_suffix',
+            'XI123456789_invalid_suffix',
         ];
 
         foreach ($invalid as $format) {