import re


def GetFramePlaceholder(frameFormat):
    """
    Gets the frame placeholder in a frame format string.

    This function expects the input frameFormat string to contain exactly one
    frame placeholder. The placeholder must be composed of exactly one or two
    groups of one or more hashes ('#'), and if there are two, they must be
    separated by a dot ('.').

    If no such placeholder exists in the frame format string, None is returned.
    """
    if not frameFormat:
        return None


    PLACEHOLDER_PATTERN = r'^[^#]*(?P<placeholder>#+(\.#+)?)[^#]*$'

    matches = re.search(PLACEHOLDER_PATTERN, frameFormat)
    if not matches:
        return None

    placeholder = matches.group(1)

    return placeholder