forms.py

from django.core.exceptions import ValidationError
from django import forms


class UsernameValidatorForm(forms.Form):
    username = forms.CharField()

    def clean_username(self):
        username = self.cleaned_data['username']

        ## Character pattern, or regex, set to 'characters' for use later to make sure username contains only letters or digits with one optional space anywhere in between, and begins and ends with only letters or digits.
        characters = re.compile(r'^[\d|\w]+[\s{1}]?[\d|\w]+$')
        if not re.match(characters, username):
            raise ValidationError("Usernames can only contain characters ...")

        ## Get 'username' length to test later since usernames need to be checked for both having only the correct characters AND to be of the correct length.
        length = len(username)
        ## Valid returns true?  Then check to see if size requirements are met, modifying 'valid' if not.
        if not (8 <= length <= 16):
            raise ValidationError("Usernames must be between 8 and 16 characters long")

        return username

views.py

from django.shortcuts import redirect, render

from .forms import UsernameValidatorForm
from .models import User


def index(request, username = None):
    if request.method == 'POST':
        # The visitor has submitted a form! Hurrah
        form = UsernameValidatorForm(request.POST)

        # Is the data in the form valid?
        if form.is_valid():
            # The form is valid! Great success. Do something with the data
            pass

            # Now redirect the user somewhere useful
            return redirect('Name:success')
        else:
            # Bad luck, show an error message
            messages.error(request, 'There was an error validating your username')
    else:
        # Make an empty form for new visitors
        form = UsernameValidatorForm()

    ## Make sure messages gets added to 'render()' once templates and routing are complete.
    return render(request, 'name/index.html', {
        'form': form,
    })