import re


def is_valid_pred_string(predstr):
    """
    Return `True` if the given predicate string represents a valid
    Pred, `False` otherwise.
    """
    predstr = predstr.strip('"').lstrip("'")
    # this is a stricter regex than in Pred, but doesn't check POS
    return re.match(
        r'_([^ _\\]|\\.)+_[a-z](_([^ _\\]|\\.)+)?(_rel)?$'
        r'|[^_]([^ \\]|\\.)+(_rel)?$',
        predstr
    ) is not None