Yes during debugging it shows u have to put it in parenthesis otherwise it fails. Ask @otcar for details 

"Norman Jäckel" <notifications@github.com> schrieb:
>> @@ -23,7 +23,7 @@ class ItemForm(CleanHtmlFormMixin, CssClassMixin,
>forms.ModelForm):
>>          queryset=Item.objects.all(), label=ugettext_lazy("Parent
>item"), required=False)
>>  
>>      duration = forms.RegexField(
>> -        regex=re.compile('[0-99]:[0-5][0-9]'),
>> +        regex=re.compile('^(?:[0-9]{1,2}:[0-5][0-9]|[0-9]+)$'),
>
>Is it necessary to put the regex into a group? Can we also use
>`regex=re.compile('^[0-9]{1,2}:[0-5][0-9]|[0-9]+$')`?
>
>---
>Reply to this email directly or view it on GitHub:
>https://github.com/OpenSlides/OpenSlides/pull/1087/files#r7789511

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