import re


def vet_pdb_id(pdbid):
    """PDB ID must be exactly four characters long, alphanumeric, and
    the first character must be an integer.
    """
    if len(pdbid) < 4 or not \
       pdbid.isalnum() or not \
       re.match(r'^[0-9][A-Za-z0-9]{3}$', pdbid):
        return False
    return True