python - nltk 正则表达式分词器

Question

我尝试在 python 中使用 nltk 实现正则表达式分词器，但结果是这样的:

>>> import nltk
>>> text = 'That U.S.A. poster-print costs $12.40...'
>>> pattern = r'''(?x)    # set flag to allow verbose regexps
...     ([A-Z]\.)+        # abbreviations, e.g. U.S.A.
...   | \w+(-\w+)*        # words with optional internal hyphens
...   | \$?\d+(\.\d+)?%?  # currency and percentages, e.g. $12.40, 82%
...   | \.\.\.            # ellipsis
...   | [][.,;"'?():-_`]  # these are separate tokens; includes ], [
... '''
>>> nltk.regexp_tokenize(text, pattern)
[('', '', ''), ('', '', ''), ('', '-print', ''), ('', '', ''), ('', '', '')]

但想要的结果是这样的:

['That', 'U.S.A.', 'poster-print', 'costs', '$12.40', '...']

为什么？哪里错了？

Answer 1

你应该把所有的捕获组都变成非捕获组:

• ([A-Z]\.)+ > (?:[A-Z]\.)+
• \w+(-\w+)* -> \w+(?:-\w+)*
• \$?\d+(\.\d+)?%? 到 \$?\d+(?:\.\d+)?%?<

问题是 regexp_tokenize 似乎正在使用 re.findall，当模式中定义了多个捕获组时，它会返回捕获元组列表。参见 this nltk.tokenize package reference :

pattern (str) – The pattern used to build this tokenizer. (This pattern must not contain capturing parentheses; Use non-capturing parentheses, e.g. (?:...), instead)

此外，我不确定您是否想使用匹配包含所有大写字母的范围的 :-_，将 - 放在字符类的末尾。

因此，使用

pattern = r'''(?x)          # set flag to allow verbose regexps
        (?:[A-Z]\.)+        # abbreviations, e.g. U.S.A.
      | \w+(?:-\w+)*        # words with optional internal hyphens
      | \$?\d+(?:\.\d+)?%?  # currency and percentages, e.g. $12.40, 82%
      | \.\.\.              # ellipsis
      | [][.,;"'?():_`-]    # these are separate tokens; includes ], [
    '''