52 lines
2.2 KiB
Plaintext
52 lines
2.2 KiB
Plaintext
[[analyzer-anatomy]]
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=== Anatomy of an analyzer
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An _analyzer_ -- whether built-in or custom -- is just a package which
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contains three lower-level building blocks: _character filters_,
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_tokenizers_, and _token filters_.
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The built-in <<analysis-analyzers,analyzers>> pre-package these building
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blocks into analyzers suitable for different languages and types of text.
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Elasticsearch also exposes the individual building blocks so that they can be
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combined to define new <<analysis-custom-analyzer,`custom`>> analyzers.
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==== Character filters
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A _character filter_ receives the original text as a stream of characters and
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can transform the stream by adding, removing, or changing characters. For
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instance, a character filter could be used to convert Hindu-Arabic numerals
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(٠١٢٣٤٥٦٧٨٩) into their Arabic-Latin equivalents (0123456789), or to strip HTML
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elements like `<b>` from the stream.
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An analyzer may have *zero or more* <<analysis-charfilters,character filters>>,
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which are applied in order.
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==== Tokenizer
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A _tokenizer_ receives a stream of characters, breaks it up into individual
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_tokens_ (usually individual words), and outputs a stream of _tokens_. For
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instance, a <<analysis-whitespace-tokenizer,`whitespace`>> tokenizer breaks
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text into tokens whenever it sees any whitespace. It would convert the text
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`"Quick brown fox!"` into the terms `[Quick, brown, fox!]`.
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The tokenizer is also responsible for recording the order or _position_ of
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each term and the start and end _character offsets_ of the original word which
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the term represents.
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An analyzer must have *exactly one* <<analysis-tokenizers,tokenizer>>.
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==== Token filters
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A _token filter_ receives the token stream and may add, remove, or change
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tokens. For example, a <<analysis-lowercase-tokenfilter,`lowercase`>> token
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filter converts all tokens to lowercase, a
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<<analysis-stop-tokenfilter,`stop`>> token filter removes common words
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(_stop words_) like `the` from the token stream, and a
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<<analysis-synonym-tokenfilter,`synonym`>> token filter introduces synonyms
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into the token stream.
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Token filters are not allowed to change the position or character offsets of
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each token.
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An analyzer may have *zero or more* <<analysis-tokenfilters,token filters>>,
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which are applied in order. |