import re


def raw_seconds_short(string):
    """Formats a human-readable M:SS string as a float (number of seconds).

    Raises ValueError if the conversion cannot take place due to `string` not
    being in the right format.
    """
    match = re.match(r'^(\d+):([0-5]\d)$', string)
    if not match:
        raise ValueError(u'String not in M:SS format')
    minutes, seconds = map(int, match.groups())
    return float(minutes * 60 + seconds)