gistfile1.py

from django.db import models
from django import forms
from django.utils.translation import ugettext_lazy as _

class HexField(models.CharField):
    def __init__(self, *args, **kwargs):
        self.min_length = kwargs.pop('min_length', None)
        super(HexField, self).__init__(*args, **kwargs)
        
    def formfield(self, **kwargs):
        defaults = {
            'form_class': forms.RegexField,
            'regex': '^[A-Fa-f0-9]+$',
            'max_length': self.max_length,
            'min_length': self.min_length,
            'error_messages': {
                'invalid': _(u'Enter only valid hexadecimal characters.')
            }
        }
        defaults.update(kwargs)
        return super(HexField, self).formfield(**defaults)