import re


def find_single_letters(question):
    """
    if the word "letter" (and plural "letters") is mentioned, return all single letters
    """
    if re.findall(r"\bletter\b|\bletters\b", question):
        matches = re.findall(r"\b[A-Za-z]\b", question)

        return [m for m in matches]

    return []