import re

componentRegex = re.compile(r"^[a-z][a-z0-9_]*$")


def checkURI(uri):
    """
    uri: lowercase, dot separated string.
    throws exception if invalid.
    returns: uri
    """

    components = uri.split(".")
    for component in components:
        match = componentRegex.match(component)
        if not match:
            msg = "invalid URI"
            raise Exception(msg)
    return uri