def string_slice(strvar,slicevar):
    """ slice a string with |string_slice:'[first]:[last]'
    """
    first,last= slicevar.partition(':')[::2]
    if first=='':
        return strvar[:int(last)]
    elif last=='':
        return strvar[int(first):]
    else:
        return strvar[int(first):int(last)]