diff --git a/README.md b/README.md
index f862212..1b46378 100644
--- a/README.md
+++ b/README.md
@@ -63,12 +63,19 @@ When given a string, it interpolates the string with the following fields:
 * %f - first @@n1_length characters of number (configured through Phoner::Phone.n1_length), default is 3 (512)
 * %l - last characters of number (5486)
 * %x - the extension number
+* %d{} - characters in brackets will only appear if extension is present
 
 ```ruby
 pn = Phoner::Phone.parse('+385915125486')
 pn.to_s # => "+385915125486"
 pn.format("%A/%f-%l") # => "091/512-5486"
 pn.format("+ %c (%a) %n") # => "+ 385 (91) 5125486"
+pn.format("+ %c (%a) %n%d{ ext.}%x") # => "+ 385 (91) 5125486"
+```
+```ruby
+pn = Phoner::Phone.parse('+1-800-555-0125x443')
+pn.format("%c (%a) %n") # => "1 (800) 5550125"
+pn.format("%c (%a) %n%d{ ext.}%x") # => "1 (800) 5550125 ext.443"
 ```
 
 When given a symbol it is used as a lookup for the format in the <tt>Phoner::Phone.named_formats</tt> hash.
@@ -77,6 +84,7 @@ When given a symbol it is used as a lookup for the format in the <tt>Phoner::Pho
 pn.format(:europe) # => "+385 (0) 91 512 5486"
 pn.format(:us) # => "(234) 123-4567"
 pn.format(:default_with_extension) # => "+3851234567x143"
+
 ```
 
 You can add your own custom named formats like so:
diff --git a/lib/phone.rb b/lib/phone.rb
index e5169e9..6d79225 100644
--- a/lib/phone.rb
+++ b/lib/phone.rb
@@ -338,6 +338,8 @@ def format_number(fmt)
       # TODO: When we drop 1.8.7 we can pass the hash in as an arg to #gsub
       fmt.gsub(FORMAT_TOKENS) do |match|
         replacements[match.to_s]
+      end.gsub(/%d\{(.*?)\}/) do
+        extension.nil? ? '' : $1
       end
     end
   end
diff --git a/test/phone_test.rb b/test/phone_test.rb
index 638f366..a678a9d 100644
--- a/test/phone_test.rb
+++ b/test/phone_test.rb
@@ -145,6 +145,18 @@ def test_format_with_symbol_specifier
     assert_equal pn.format(:europe), '+385 (0) 91 512 5486'
   end
 
+  def test_format_with_conditional_extension_with_extension
+    Phoner::Phone.default_country_code = nil
+    pn = Phoner::Phone.new '5125486', '191', '385', '242'
+    assert_equal pn.format('+ %c (%a) %n%d{ #}%x'), '+ 385 (191) 5125486 #242'
+  end
+
+  def test_format_with_conditional_extension_without_extension
+    Phoner::Phone.default_country_code = nil
+    pn = Phoner::Phone.new '5125486', '191', '385'
+    assert_equal pn.format('+ %c (%a) %n%d{ #}%x'), '+ 385 (191) 5125486'
+  end
+
   def test_validity
     assert true, Phoner::Phone.valid?("+17788827175")
   end