Regex for form keys.

[DrLulz]

I have a form which contains a variable number of fields notated as p1_value, p2_value, p3_value, etc. (e.g. sometimes there may only be p1 and p2, yet other times they may be a p4). How can I represent this when validating the form? Thank you.

form = {
        '_value': [
            Regexp(SOME_PATTERN, message=u'MESSAGE')
        ]
    }