import re


def get_isotopes(ratio_text):
    """
    Regex for isotope ratios.

    Parameters
    -----------
    ratio_text : :class:`str`
        Text to extract isotope ratio components from.

    Returns
    -----------
    :class:`list`
        Isotope ration numerator and denominator.
    """
    forward_isotope = r"([a-zA-Z][a-zA-Z]?[0-9][0-9]?[0-9]?)"
    backward_isotope = r"([0-9][0-9]?[0-9]?[a-zA-Z][a-zA-Z]?)"
    fw = re.findall(forward_isotope, ratio_text)
    bw = re.findall(backward_isotope, ratio_text)
    lfw, lbw = len(fw), len(bw)
    if (lfw > 1 and lbw > 1) or ((lfw < 2) and (lbw < 2)):
        return []
    elif lfw == 2:
        return fw
    elif lbw == 2:
        return bw