diff --git a/ox/html.py b/ox/html.py
index 815f657..13524fd 100644
--- a/ox/html.py
+++ b/ox/html.py
@@ -6,8 +6,10 @@ import string
 from six.moves.html_entities import name2codepoint
 from six import unichr, PY2, string_types
 
+letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
 
 # Configuration for add_links() function
+
 LEADING_PUNCTUATION  = ['(', '<', '&lt;']
 TRAILING_PUNCTUATION = ['.', ',', ')', '>', '\n', '&gt;', "'", '"']
 
@@ -89,7 +91,7 @@ def add_links(text, trim_url_limit=None, nofollow=False):
         if match:
             lead, middle, trail = match.groups()
             if middle.startswith('www.') or ('@' not in middle and not middle.startswith('http://') and \
-                    len(middle) > 0 and middle[0] in string.letters + string.digits and \
+                    len(middle) > 0 and middle[0] in letters + string.digits and \
                     (middle.endswith('.org') or middle.endswith('.net') or middle.endswith('.com'))):
                 middle = '<a href="http://%s"%s>%s</a>' % (middle, nofollow_attr, trim_url(middle))
             if middle.startswith('http://') or middle.startswith('https://'):