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51 lines
2.3 KiB
Markdown
51 lines
2.3 KiB
Markdown
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During processing, spaCy first **tokenizes** the text, i.e. segments it into
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words, punctuation and so on. This is done by applying rules specific to each
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language. For example, punctuation at the end of a sentence should be split off
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– whereas "U.K." should remain one token. Each `Doc` consists of individual
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tokens, and we can iterate over them:
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```python
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### {executable="true"}
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import spacy
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nlp = spacy.load("en_core_web_sm")
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doc = nlp(u"Apple is looking at buying U.K. startup for $1 billion")
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for token in doc:
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print(token.text)
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```
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| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
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| :---: | :-: | :-----: | :-: | :----: | :--: | :-----: | :-: | :-: | :-: | :-----: |
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| Apple | is | looking | at | buying | U.K. | startup | for | \$ | 1 | billion |
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First, the raw text is split on whitespace characters, similar to
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`text.split(' ')`. Then, the tokenizer processes the text from left to right. On
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each substring, it performs two checks:
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1. **Does the substring match a tokenizer exception rule?** For example, "don't"
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does not contain whitespace, but should be split into two tokens, "do" and
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"n't", while "U.K." should always remain one token.
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2. **Can a prefix, suffix or infix be split off?** For example punctuation like
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commas, periods, hyphens or quotes.
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If there's a match, the rule is applied and the tokenizer continues its loop,
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starting with the newly split substrings. This way, spaCy can split **complex,
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nested tokens** like combinations of abbreviations and multiple punctuation
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marks.
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> - **Tokenizer exception:** Special-case rule to split a string into several
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> tokens or prevent a token from being split when punctuation rules are
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> applied.
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> - **Prefix:** Character(s) at the beginning, e.g. `$`, `(`, `“`, `¿`.
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> - **Suffix:** Character(s) at the end, e.g. `km`, `)`, `”`, `!`.
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> - **Infix:** Character(s) in between, e.g. `-`, `--`, `/`, `…`.
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While punctuation rules are usually pretty general, tokenizer exceptions
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strongly depend on the specifics of the individual language. This is why each
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[available language](/usage/models#languages) has its own subclass like
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`English` or `German`, that loads in lists of hard-coded data and exception
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rules.
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