def parse_string(progression):
    """Return a tuple (roman numeral, accidentals, chord suffix).

    Examples:
    >>> parse_string('I')
    ('I', 0, '')
    >>> parse_string('bIM7')
    ('I', -1, 'M7')
    """
    acc = 0
    roman_numeral = ''
    suffix = ''
    i = 0
    for c in progression:
        if c == '#':
            acc += 1
        elif c == 'b':
            acc -= 1
        elif c.upper() == 'I' or c.upper() == 'V':
            roman_numeral += c.upper()
        else:
            break
        i += 1
    suffix = progression[i:]
    return roman_numeral, acc, suffix
