Define a custom form for the model administrator and override the specific field to use RegexField.
class MyModel(models.Model):
myfield = models.CharField(max_length=10)
class MyModelForm(forms.ModelForm):
myfield = forms.RegexField(regex=r'\w+')
class MyModelAdmin(admin.ModelAdmin):
form = MyModelForm
admin.site.register(MyModel, MyModelAdmin)
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