Add preprocessing step for transfo-xl tokenization to avoid tokenizing words followed by punction to <unk> (#2987)

* add preprocessing to add space before punctuation for transfo_xl

* improve warning messages

* make style

* compile regex at instantination of tokenizer object

examples/run_generation.py

@@ -59,7 +59,7 @@ MODEL_CLASSES = {
 # Padding text to help Transformer-XL and XLNet with short prompts as proposed by Aman Rusia
 # in https://github.com/rusiaaman/XLNet-gen#methodology
 # and https://medium.com/@amanrusia/xlnet-speaks-comparison-to-gpt-2-ea1a4e9ba39e
-PADDING_TEXT = """ In 1991, the remains of Russian Tsar Nicholas II and his family
+PADDING_TEXT = """In 1991, the remains of Russian Tsar Nicholas II and his family
 (except for Alexei and Maria) are discovered.
 The voice of Nicholas's young son, Tsarevich Alexei Nikolaevich, narrates the
 remainder of the story. 1883 Western Siberia,
@@ -214,7 +214,9 @@ def main():
     if requires_preprocessing:
         prepare_input = PREPROCESSING_FUNCTIONS.get(args.model_type)
         preprocessed_prompt_text = prepare_input(args, model, tokenizer, prompt_text)
-        encoded_prompt = tokenizer.encode(preprocessed_prompt_text, add_special_tokens=False, return_tensors="pt")
+        encoded_prompt = tokenizer.encode(
+            preprocessed_prompt_text, add_special_tokens=False, return_tensors="pt", add_space_before_punct_symbol=True
+        )
     else:
         encoded_prompt = tokenizer.encode(prompt_text, add_special_tokens=False, return_tensors="pt")
     encoded_prompt = encoded_prompt.to(args.device)

src/transformers/tokenization_transfo_xl.py

@@ -22,6 +22,7 @@ import glob
 import logging
 import os
 import pickle
+import re
 from collections import Counter, OrderedDict
 from typing import List, Optional, Tuple, Union
 
@@ -114,6 +115,9 @@ class TransfoXLTokenizer(PreTrainedTokenizer):
         self.delimiter = delimiter
         self.vocab_file = vocab_file
         self.never_split = never_split
+        self.punctuation_symbols = '!"#$%&()*+,-./\:;<=>?@[\\]^_`{|}~'  # noqa: W605
+        self.punction_without_space_before_pattern = re.compile(r"[^\s][{}]".format(self.punctuation_symbols))
+        self.punctuation_with_space_around_pattern = self._compile_space_around_punctuation_pattern()
 
         if pretrained_vocab_file is not None:
             # Hack because, honestly this tokenizer was not made to be used
@@ -126,6 +130,11 @@ class TransfoXLTokenizer(PreTrainedTokenizer):
         if vocab_file is not None:
             self.build_vocab()
 
+    def _compile_space_around_punctuation_pattern(self):
+        look_ahead_for_special_token = "(?=[{}])".format(self.punctuation_symbols)
+        look_ahead_to_match_all_except_space = "(?=[^\s])"  # noqa: W605
+        return re.compile(r"" + look_ahead_for_special_token + look_ahead_to_match_all_except_space)
+
     def count_file(self, path, verbose=False, add_eos=False):
         if verbose:
             logger.info("counting file {} ...".format(path))
@@ -295,6 +304,19 @@ class TransfoXLTokenizer(PreTrainedTokenizer):
         else:
             return symbols
 
+    def prepare_for_tokenization(self, text, **kwargs):
+        # add spaces before punctuation symbols as should be done in transfo-xl
+
+        if "add_space_before_punct_symbol" in kwargs and kwargs["add_space_before_punct_symbol"]:
+            text = self.punctuation_with_space_around_pattern.sub(r" ", text)
+        elif self.punction_without_space_before_pattern.search(text):
+            # searches until the first occurence of a punctuation symbol without surrounding spaces
+            logger.warning(
+                "You might want to consider setting `add_space_before_punct_symbol=True` as an argument to the `tokenizer.encode()` to avoid tokenizing words with punctuation symbols to the `<unk>` token"
+            )
+
+        return text
+
 
 class _TransfoXLDelimiterLookupTokenizer(BaseTokenizer):
     def __init__(