diff --git a/lib/sepa_king/transaction/direct_debit_transaction.rb b/lib/sepa_king/transaction/direct_debit_transaction.rb
index 569fed0..277d003 100644
--- a/lib/sepa_king/transaction/direct_debit_transaction.rb
+++ b/lib/sepa_king/transaction/direct_debit_transaction.rb
@@ -3,7 +3,7 @@ module SEPA
   class DirectDebitTransaction < Transaction
     attr_accessor :mandate_id, :mandate_date_of_signature, :local_instrument, :sequence_type, :creditor_account
 
-    validates_length_of :mandate_id, within: 1..35
+    validates_format_of :mandate_id, :with => /\A([A-Za-z0-9]|[\+|\?|\/|\-|\:|\(|\)|\.|\,|\']){1,35}\z/
     validates_presence_of :mandate_date_of_signature
     validates_inclusion_of :local_instrument, :in => %w(CORE COR1 B2B)
     validates_inclusion_of :sequence_type, :in => %w(FRST OOFF RCUR FNAL)
diff --git a/spec/direct_debit_transaction_spec.rb b/spec/direct_debit_transaction_spec.rb
index 319a9bd..cf23a8f 100644
--- a/spec/direct_debit_transaction_spec.rb
+++ b/spec/direct_debit_transaction_spec.rb
@@ -48,11 +48,11 @@
 
   context 'Mandate ID' do
     it 'should allow valid value' do
-      SEPA::DirectDebitTransaction.should accept('XYZ-123', 'X' * 35, for: :mandate_id)
+      SEPA::DirectDebitTransaction.should accept('XYZ-123', "+?/-:().,'", 'X' * 35, for: :mandate_id)
     end
 
     it 'should not allow invalid value' do
-      SEPA::DirectDebitTransaction.should_not accept(nil, '', 'X' * 36, for: :mandate_id)
+      SEPA::DirectDebitTransaction.should_not accept(nil, '', 'X' * 36, 'ABC 123', '#/*', 'Ümläüt', for: :mandate_id)
     end
   end
 end