# Buggy code
def tokenize(self, tokens, return_str=False):
    """
    Python port of the Moses detokenizer.

    :param tokens: A list of strings, i.e. tokenized text.
    :type tokens: list(str)
    :return: str
    """
    # Convert the list of tokens into a string and pad it with spaces.
    text = " {} ".format(" ".join(tokens))
    # Converts input string into unicode.
    text = text_type(text)
    # Detokenize the agressive hyphen split.
    regexp, subsitution = self.AGGRESSIVE_HYPHEN_SPLIT
    text = re.sub(regexp, subsitution, text)
    # Unescape the XML symbols.
    text = self.unescape_xml(text)
    # Keep track of no. of quotation marks.
    quote_counts = {"'": 0, '"': 0, "``": 0, "`": 0, "''": 0}

    # The *prepend_space* variable is used to control the "effects" of
    # detokenization as the function loops through the list of tokens and
    # changes the *prepend_space* accordingly as it sequentially checks
    # through the language specific and language independent conditions.
    prepend_space = " "
    detokenized_text = ""
    tokens = text.split()
    # Iterate through every token and apply language specific detokenization rule(s).
    for i, token in enumerate(iter(tokens)):
        # Check if the first char is CJK.
        if is_cjk(token[0]):
            # Perform left shift if this is a second consecutive CJK word.
            if i > 0 and is_cjk(token[-1]):
                detokenized_text += token
            # But do nothing special if this is a CJK word that doesn't follow a CJK word
            else:
                detokenized_text += prepend_space + token
            prepend_space = " "

        # If it's a currency symbol.
        elif token in self.IsSc:
            # Perform right shift on currency and other random punctuation items
            detokenized_text += prepend_space + token
            prepend_space = ""

        elif re.match(r"^[\,\.\?\!\:\;\\\%\}\]\)]+$", token):
            # In French, these punctuations are prefixed with a non-breakable space.
            if self.lang == "fr" and re.match(r"^[\?\!\:\;\\\%]$", token):
                detokenized_text += " "
            # Perform left shift on punctuation items.
            detokenized_text += token
            prepend_space = " "

        elif (
            self.lang == "en"
            and i > 0
            and re.match("^['][{}]".format(self.IsAlpha), token)
            and re.match("[{}]".format(self.IsAlnum), token)
        ):
            # For English, left-shift the contraction.
            detokenized_text += token
            prepend_space = " "

        elif (
            self.lang == "cs"
            and i > 1
            and re.match(
                r"^[0-9]+$", tokens[-2]
            )  # If the previous previous token is a number.
            and re.match(r"^[.,]$", tokens[-1])  # If previous token is a dot.
            and re.match(r"^[0-9]+$", token)
        ):  # If the current token is a number.
            # In Czech, left-shift floats that are decimal numbers.
            detokenized_text += token
            prepend_space = " "

        elif (
            self.lang in ["fr", "it"]
            and i <= len(tokens) - 2
            and re.match("[{}][']$".format(self.IsAlpha), token)
            and re.match("^[{}]$".format(self.IsAlpha), tokens[i + 1])
        ):  # If the next token is alpha.
            # For French and Italian, right-shift the contraction.
            detokenized_text += prepend_space + token
            prepend_space = ""

        elif (
            self.lang == "cs"
            and i <= len(tokens) - 3
            and re.match("[{}][']$".format(self.IsAlpha), token)
            and re.match("^[-–]$", tokens[i + 1])
            and re.match("^li$|^mail.*", tokens[i + 2], re.IGNORECASE)
        ):  # In Perl, ($words[$i+2] =~ /^li$|^mail.*/i)
            # In Czech, right-shift "-li" and a few Czech dashed words (e.g. e-mail)
            detokenized_text += prepend_space + token + tokens[i + 1]
            next(tokens, None)  # Advance over the dash
            prepend_space = ""

        # Combine punctuation smartly.
        elif re.match(r"""^[\'\"„“`]+$""", token):
            normalized_quo = token
            if re.match(r"^[„“”]+$", token):
                normalized_quo = '"'
            quote_counts.get(normalized_quo, 0)

            if self.lang == "cs" and token == "„":
                quote_counts[normalized_quo] = 0
            if self.lang == "cs" and token == "“":
                quote_counts[normalized_quo] = 1

            if quote_counts[normalized_quo] % 2 == 0:
                if (
                    self.lang == "en"
                    and token == "'"
                    and i > 0
                    and re.match(r"[s]$", tokens[i - 1])
                ):
                    # Left shift on single quote for possessives ending
                    # in "s", e.g. "The Jones' house"
                    detokenized_text += token
                    prepend_space = " "
                else:
                    # Right shift.
                    detokenized_text += prepend_space + token
                    prepend_space = ""
                    quote_counts[normalized_quo] += 1
            else:
                # Left shift.
                text += token
                prepend_space = " "
                quote_counts[normalized_quo] += 1

        elif (
            self.lang == "fi"
            and re.match(r":$", tokens[i - 1])
            and re.match(self.FINNISH_REGEX, token)
        ):
            # Finnish : without intervening space if followed by case suffix
            # EU:N EU:n EU:ssa EU:sta EU:hun EU:iin ...
            detokenized_text += prepend_space + token
            prepend_space = " "

        else:
            detokenized_text += prepend_space + token
            prepend_space = " "

    # Merge multiple spaces.
    regexp, subsitution = self.ONE_SPACE
    detokenized_text = re.sub(regexp, subsitution, detokenized_text)
    # Removes heading and trailing spaces.
    detokenized_text = detokenized_text.strip()

    return detokenized_text if return_str else detokenized_text.split()


# Traceback (sre_constants.error)
$ python -c 'from nltk.tokenize.moses import MosesTokenizer; m = MosesTokenizer(); m.penn_tokenize("this aint funny")'
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "nltk/tokenize/moses.py", line 299, in penn_tokenize
text = re.sub(regexp, subsitution, text)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 155, in sub
return _compile(pattern, flags).sub(repl, string, count)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 251, in _compile
raise error, v # invalid expression
sre_constants.error: unbalanced parenthesis

# Fixed code
def tokenize(self, tokens, return_str=False):
    """
    Python port of the Moses detokenizer.

    :param tokens: A list of strings, i.e. tokenized text.
    :type tokens: list(str)
    :return: str
    """
    # Convert the list of tokens into a string and pad it with spaces.
    text = " {} ".format(" ".join(tokens))
    # Converts input string into unicode.
    text = text_type(text)
    # Detokenize the agressive hyphen split.
    regexp, substitution = self.AGGRESSIVE_HYPHEN_SPLIT
    text = re.sub(regexp, substitution, text)
    # Unescape the XML symbols.
    text = self.unescape_xml(text)
    # Keep track of no. of quotation marks.
    quote_counts = {"'": 0, '"': 0, "``": 0, "`": 0, "''": 0}

    # The *prepend_space* variable is used to control the "effects" of
    # detokenization as the function loops through the list of tokens and
    # changes the *prepend_space* accordingly as it sequentially checks
    # through the language specific and language independent conditions.
    prepend_space = " "
    detokenized_text = ""
    tokens = text.split()
    # Iterate through every token and apply language specific detokenization rule(s).
    for i, token in enumerate(iter(tokens)):
        # Check if the first char is CJK.
        if is_cjk(token[0]):
            # Perform left shift if this is a second consecutive CJK word.
            if i > 0 and is_cjk(token[-1]):
                detokenized_text += token
            # But do nothing special if this is a CJK word that doesn't follow a CJK word
            else:
                detokenized_text += prepend_space + token
            prepend_space = " "

        # If it's a currency symbol.
        elif token in self.IsSc:
            # Perform right shift on currency and other random punctuation items
            detokenized_text += prepend_space + token
            prepend_space = ""

        elif re.search(r"^[\,\.\?\!\:\;\\\%\}\]\)]+$", token):
            # In French, these punctuations are prefixed with a non-breakable space.
            if self.lang == "fr" and re.search(r"^[\?\!\:\;\\\%]$", token):
                detokenized_text += " "
            # Perform left shift on punctuation items.
            detokenized_text += token
            prepend_space = " "

        elif (
            self.lang == "en"
            and i > 0
            and re.search("^['][{}]".format(self.IsAlpha), token)
        ):
            # and re.search(u'[{}]$'.format(self.IsAlnum), tokens[i-1])):
            # For English, left-shift the contraction.
            detokenized_text += token
            prepend_space = " "

        elif (
            self.lang == "cs"
            and i > 1
            and re.search(
                r"^[0-9]+$", tokens[-2]
            )  # If the previous previous token is a number.
            and re.search(r"^[.,]$", tokens[-1])  # If previous token is a dot.
            and re.search(r"^[0-9]+$", token)
        ):  # If the current token is a number.
            # In Czech, left-shift floats that are decimal numbers.
            detokenized_text += token
            prepend_space = " "

        elif (
            self.lang in ["fr", "it"]
            and i <= len(tokens) - 2
            and re.search("[{}][']$".format(self.IsAlpha), token)
            and re.search("^[{}]$".format(self.IsAlpha), tokens[i + 1])
        ):  # If the next token is alpha.
            # For French and Italian, right-shift the contraction.
            detokenized_text += prepend_space + token
            prepend_space = ""

        elif (
            self.lang == "cs"
            and i <= len(tokens) - 3
            and re.search("[{}][']$".format(self.IsAlpha), token)
            and re.search("^[-–]$", tokens[i + 1])
            and re.search("^li$|^mail.*", tokens[i + 2], re.IGNORECASE)
        ):  # In Perl, ($words[$i+2] =~ /^li$|^mail.*/i)
            # In Czech, right-shift "-li" and a few Czech dashed words (e.g. e-mail)
            detokenized_text += prepend_space + token + tokens[i + 1]
            next(tokens, None)  # Advance over the dash
            prepend_space = ""

        # Combine punctuation smartly.
        elif re.search(r"""^[\'\"„“`]+$""", token):
            normalized_quo = token
            if re.search(r"^[„“”]+$", token):
                normalized_quo = '"'
            quote_counts.get(normalized_quo, 0)

            if self.lang == "cs" and token == "„":
                quote_counts[normalized_quo] = 0
            if self.lang == "cs" and token == "“":
                quote_counts[normalized_quo] = 1

            if quote_counts[normalized_quo] % 2 == 0:
                if (
                    self.lang == "en"
                    and token == "'"
                    and i > 0
                    and re.search(r"[s]$", tokens[i - 1])
                ):
                    # Left shift on single quote for possessives ending
                    # in "s", e.g. "The Jones' house"
                    detokenized_text += token
                    prepend_space = " "
                else:
                    # Right shift.
                    detokenized_text += prepend_space + token
                    prepend_space = ""
                    quote_counts[normalized_quo] += 1
            else:
                # Left shift.
                text += token
                prepend_space = " "
                quote_counts[normalized_quo] += 1

        elif (
            self.lang == "fi"
            and re.search(r":$", tokens[i - 1])
            and re.search(self.FINNISH_REGEX, token)
        ):
            # Finnish : without intervening space if followed by case suffix
            # EU:N EU:n EU:ssa EU:sta EU:hun EU:iin ...
            detokenized_text += prepend_space + token
            prepend_space = " "

        else:
            detokenized_text += prepend_space + token
            prepend_space = " "

    # Merge multiple spaces.
    regexp, substitution = self.ONE_SPACE
    detokenized_text = re.sub(regexp, substitution, detokenized_text)
    # Removes heading and trailing spaces.
    detokenized_text = detokenized_text.strip()

    return detokenized_text if return_str else detokenized_text.split()