import re

examples = [
    "D1",       # expected: ('1')
    "D1sjdgf",  # ('1')
    "D1.2",     # ('1', '2')
    "D1.2.3",   # ('1', '2', '3')
    "D3.10.3x", # ('3', '10', '3')
    "D3.10.11"  # ('3', '10', '11')
]

rx = re.compile(r'^D(\d+)(?:\.(\d+)(?:\.(\d+))?)?')

for s in examples:
    result = rx.search(s)
    print(s, result.groups())