Better regex expression for extracting language code in tokenization_marian.py
# Better regex expression for extracting language code in tokenization_marian.py

In `tokenization_marian.py`, the regex `>>.+<<` is used to extract the language token from the sentences leading to the following incorrect tokenization.

```
Example Sentence:
>>hin<< We use cout<< function to print a line in C++.

Current Tokenizer gives:
['<unk>', '▁function', '▁to', '▁print', '▁a', '▁line', '▁in', '▁C', '++', '.', '</s>']

Expected Tokenization:
['>>hin<<', '▁We', '▁use', '▁c', 'out', '<', '<', '▁function', '▁to', '▁print', '▁a', '▁line', '▁in', '▁C', '++', '.', '</s>']
```

This pull request changes the regex to `>>.{3}<<|>>.{3}\_.{4}<<` which covers the 194 language tags in the en-mul model.

@sshleifer 
cc @patrickvonplaten <|||||>No, 2 character language codes are not covered. However `>>.{2}<<|>>.{3}<<|>>.{3}\_.{4}<<` can be used.

I can try writing tests. Is these somewhere where I could find an exhaustive set of language codes? I had written the regex to cover the 194 language code in `'Helsinki-NLP/opus-mt-en-mul'`.

```python
def test_language_codes(self):
    tok = MarianTokenizer.from_pretrained(f"{ORG_NAME}opus-mt-en-mul")
    batch = tok.prepare_seq2seq_batch([">>hin<< I am a small frog", ">>zlm_Latn<< I am a small frog", ">>fr<< I am a small frog"], return_tensors=FRAMEWORK)
    
    expected = [[[888, 21, 437, 9,  2613, 37565, 0], [770, 21, 437, 9, 2613, 37565, 0], [1, 21, 437, 9, 2613, 37565, 0]]
    for i in range(3):
        self.assertListEqual(expected[i], batch.input_ids[i])
```

The above will need some changes since `>>fr<<` is not in the vocabulary and the sentences also do not have another `<<` in them (maybe a different test for that?).<|||||>> Is there somewhere where I could find an exhaustive set of language codes?

Not easily, you can find the supported codes for any model by looking for `tgt_languages` or `tgt_constituents` on a model card e.g. https://huggingface.co/Helsinki-NLP/opus-mt-en-ROMANCE or https://huggingface.co/Helsinki-NLP/opus-mt-en-roa?text=%3E%3Efra%3C%3C+My+name+is+Sarah+and+I+live+in+London


Here are some more:

```python
tgt_constituents= {'ita', 'cat', 'roh', 'spa', 'pap', 'lmo', 'mwl', 'lij', 'lad_Latn', 'ext', 'ron', 'ast', 'glg', 'pms', 'zsm_Latn', 'gcf_Latn', 'lld_Latn', 'min', 'tmw_Latn', 'cos', 'wln', 'zlm_Latn', 'por', 'egl', 'oci', 'vec', 'arg', 'ind', 'fra', 'hat', 'lad', 'max_Latn', 'frm_Latn', 'scn', 'mfe'}
tgt_constituents: {'cmn_Hans', 'nan', 'nan_Hani', 'gan', 'yue', 'cmn_Kana', 'yue_Hani', 'wuu_Bopo', 'cmn_Latn', 'yue_Hira', 'cmn_Hani', 'cjy_Hans', 'cmn', 'lzh_Hang', 'lzh_Hira', 'cmn_Hant', 'lzh_Bopo', 'zho', 'zho_Hans', 'zho_Hant', 'lzh_Hani', 'yue_Hang', 'wuu', 'yue_Kana', 'wuu_Latn', 'yue_Bopo', 'cjy_Hant', 'yue_Hans', 'lzh', 'cmn_Hira', 'lzh_Yiii', 'lzh_Hans', 'cmn_Bopo', 'cmn_Hang', 'hak_Hani', 'cmn_Yiii', 'yue_Hant', 'lzh_Kana', 'wuu_Hani'}
```


I think we should accept any number of alphabet characters or '_' inside the `>>` or at least up to 8.



<|||||>This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

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