def which_date(text):
    usemdy = re.findall("<ref>[^<]+\|\s*date\s*=[^|]+\d+,?\s*\\d{4}", text)
    useymd = re.findall("<ref>[^<]+\|\s*date\s*=\s*\d{4}-\d{2}-\d{2}", text)
    usedmy = re.findall("<ref>[^<]+\|\s*date\s*=\s*\d+[^|]+\d{4}", text)
    if useymd >= usemdy and useymd >= usedmy: return 'useymd'
    if usemdy >= usedmy: return 'usemdy'
    return 'usedmy'

t1 = 'Wikipedia text here<ref>{{cite web|title=test|date=Sept. 5, 2006}}</ref>'
t2 = 'Text here<ref>{{cite web | date= 2006-04-01 | title=test}}</ref>'
t3 = "Text here<ref>{{cite web | date = 5 September, 2005}}</ref>"
>>> which_date(t1)
'usemdy'
>>> which_date(t2)
'useymd'
>>> which_date(t3)
'usedmy'