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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _convert_word(word):
 """Return the plural form of the word if it exists. Otherwise return the word itself. """

    out = None

    # Acronyms.
    if word.isupper():
        out = word + "s?"
    # Proper nouns or word with digits.
    elif word.istitle():
        out = word + "('?s)?"
    elif _contains_digit.search(word):
        out = word

    if out is not None:
        return out

    # Words with non or anti prefixes.
    if _starts_with_non.search(word):
        word = "non-?" + _capitalize_first_letter(_convert_word(word[3:]))
    elif _starts_with_anti.search(word):
        word = "anti-?" + _capitalize_first_letter(_convert_word(word[4:]))

    if out is not None:
        return _capitalize_first_letter(out)

    # A few invariable words.
    if word in current_app.config["CLASSIFIER_INVARIABLE_WORDS"]:
        return _capitalize_first_letter(word)

    # Some exceptions that would not produce good results with the set of
    # general_regular_expressions.
    regexes = current_app.config["CLASSIFIER_EXCEPTIONS"]
    if word in regexes:
        return _capitalize_first_letter(regexes[word])

    regexes = current_app.config["CLASSIFIER_UNCHANGE_REGULAR_EXPRESSIONS"]
    for regex in regexes:
        if regex.search(word) is not None:
            return _capitalize_first_letter(word)

    regexes = current_app.config["CLASSIFIER_GENERAL_REGULAR_EXPRESSIONS"]
    for regex, replacement in regexes:
        stemmed = regex.sub(replacement, word)
        if stemmed != word:
            return _capitalize_first_letter(stemmed)

    return _capitalize_first_letter(word + "s?")