import re


def get_version(release_id):
    """Get version from release id.

    :param release_id: str, release id, example: f33
    :return: str, version ("f33" -> "33")
    """
    m = re.match(r"^f(\d+)$", release_id)
    if not m:
        raise ValueError('Invalid release id: %s', release_id)
    return m.group(1)