import re


def validate_username(username):
    """
    Returns True if the username matches Github constraints.
    Valid usernames: oubiga, oubiga2000, oubi-ga, Oub75igA
    Invalid usernames: -oubiga, oub_iga, ou.biga
    """
    result = re.match(r"^[a-zA-Z0-9][-a-zA-Z0-9]+$", username)
    return result is not None