def extractVersion(tag):
    """
    Extract the version number from as SVN tag.

    >>> extractVersion('GAUDI_v23r8')
    'v23r8'
    >>> extractVersion('LCGCMT_preview')
    'preview'
    >>> extractVersion('HEAD')
    'HEAD'
    """
    if '_' in tag:
        return tag.split('_', 1)[1]
    return tag
