Customized the short URL input field.

go/go/forms.py

 from django import forms
-from django.forms import ModelForm, TextInput, RadioSelect, URLInput
 from go.models import URL
-from django.core.validators import MinLengthValidator, MinValueValidator
+from django.core.validators import MinLengthValidator, MinValueValidator, RegexValidator
 
-class URLForm( ModelForm ):
+class URLForm( forms.ModelForm ):
 
     DAY = '1 Day'
     WEEK = '1 Week'
@@ -23,21 +22,32 @@ class URLForm( ModelForm ):
         label = 'Expiration',
         choices = EXPIRATION_CHOICES,
         initial = NEVER,
-        widget = RadioSelect(),
+        widget = forms.RadioSelect(),
+    )
+
+    # Add a custom short field for validation.
+    alphanumeric = RegexValidator(
+        r'^[a-zA-Z]*$',
+        'Only letters are allowed.'
+    )
+    short = forms.CharField(
+        required = False,
+        label = 'Short URL (Optional)',
+        widget = forms.TextInput(attrs={}),
+        validators = [alphanumeric],
+        max_length = 20,
+        min_length = 3,
     )
 
     class Meta:
         model = URL
-        fields = ('target', 'short')
-        exclude = ('owner', 'date_created', 'clicks', 'expires')
+        fields = ('target',)
+        exclude = ('owner', 'short', 'date_created', 'clicks', 'expires')
         labels = {
             'target': 'Long URL',
-            'short': 'Short URL (Optional)',
         }
         widgets = {
-            'target': URLInput(attrs={
+            'target': forms.URLInput(attrs={
                 'placeholder': 'http://',
             }),
-            'short': TextInput(attrs={
-            }),
         }

go/go/views.py

@@ -22,7 +22,10 @@ def index(request):
             url = url_form.save(commit=False)
             url.owner = request.user
 
-            expires = url_form.cleaned_data['expires']
+            short = url_form.cleaned_data.get('short')
+            url.short = short
+
+            expires = url_form.cleaned_data.get('expires')
 
             if expires == URLForm.DAY:
                 url.expires = timezone.now() + timedelta(days=1)