Fixes issue with domains that have a single character (#122)

Author: kingbuzzman

tests/test_domain.py

@@ -10,6 +10,9 @@
     'underscore_subdomain.example.com',
     'something.versicherung',
     '11.com',
+    '3.cn',
+    'a.cn',
+    'sub1.sub2.sample.co.uk',
     'somerandomexample.xn--fiqs8s'
 ])
 def test_returns_true_on_valid_domain(value):
@@ -22,6 +25,9 @@ def test_returns_true_on_valid_domain(value):
     'example.-com',
     'example.',
     '-example.com',
+    'example-.com',
+    '_example.com',
+    'example_.com',
     'example',
     'a......b.com'
 ])

validators/domain.py

@@ -3,10 +3,10 @@
 from .utils import validator
 
 pattern = re.compile(
-    r'^(:?(([a-zA-Z]{1})|([a-zA-Z]{1}[a-zA-Z]{1})|'  # domain pt.1
-    r'([a-zA-Z]{1}[0-9]{1})|([0-9]{1}[a-zA-Z]{1})|'  # domain pt.2
-    r'([a-zA-Z0-9][-_a-zA-Z0-9]{0,61}[a-zA-Z0-9]))\.)+'  # domain pt.3
-    r'([a-zA-Z]{2,13}|(xn--[a-zA-Z0-9]{2,30}))$'  # TLD
+    r'^(?:[a-z0-9]'  # First character of the domain
+    r'(?:[a-z0-9-_]{0,61}[a-z0-9])?\.)'  # Sub domain + hostname
+    r'+[a-z0-9][a-z0-9-_]{0,61}'  # First 61 characters of the gTLD
+    r'[a-z0-9]$'  # Last character of the gTLD
 )