12.5. Parsers
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## 12.5. Parsers #

Text search parsers are responsible for splitting raw document text into tokens and identifying each token's type, where the set of possible types is defined by the parser itself. Note that a parser does not modify the text at all — it simply identifies plausible word boundaries. Because of this limited scope, there is less need for application-specific custom parsers than there is for custom dictionaries. At present PostgreSQL provides just one built-in parser, which has been found to be useful for a wide range of applications.

The built-in parser is named `pg_catalog.default`. It recognizes 23 token types, shown in Table 12.1.

Table 12.1. Default Parser's Token Types

Alias | Description | Example
--- | --- | ---
`asciiword` | Word, all ASCII letters | `elephant`
`word` | Word, all letters | `mañana`
`numword` | Word, letters and digits | `beta1`
`asciihword` | Hyphenated word, all ASCII | `up-to-date`
`hword` | Hyphenated word, all letters | `lógico-matemática`
`numhword` | Hyphenated word, letters and digits | `postgresql-beta1`
`hword_asciipart` | Hyphenated word part, all ASCII | `postgresql` in the context `postgresql-beta1`
`hword_part` | Hyphenated word part, all letters | `lógico` or `matemática` in the context `lógico-matemática`
`hword_numpart` | Hyphenated word part, letters and digits | `beta1` in the context `postgresql-beta1`
`email` | Email address | `foo@example.com`
`protocol` | Protocol head | `http://`
`url` | URL | `example.com/stuff/index.html`
`host` | Host | `example.com`
`url_path` | URL path | `/stuff/index.html`, in the context of a URL
`file` | File or path name | `/usr/local/foo.txt`, if not within a URL
`sfloat` | Scientific notation | `-1.234e56`
`float` | Decimal notation | `-1.234`
`int` | Signed integer | `-1234`
`uint` | Unsigned integer | `1234`
`version` | Version number | `8.3.0`
`tag` | XML tag | `<a href="dictionaries.html">`
`entity` | XML entity | `&amp;`
`blank` | Space symbols | (any whitespace or punctuation not otherwise recognized)

### Note

The parser's notion of a “letter” is determined by the database's locale setting, specifically `lc_ctype`. Words containing only the basic ASCII letters are reported as a separate token type, since it is sometimes useful to distinguish them. In most European languages, token types `word` and `asciiword` should be treated alike.

`email` does not support all valid email characters as defined by RFC 5322. Specifically, the only non-alphanumeric characters supported for email user names are period, dash, and underscore.

`tag` does not support all valid tag names as defined by W3C Recommendation, XML. Specifically, the only tag names supported are those starting with an ASCII letter, underscore, or colon, and containing only letters, digits, hyphens, underscores, periods, and colons. `tag` also includes XML comments starting with `<!--` and ending with `-->`, and XML declarations (but note that this includes anything starting with `<?x` and ending with `>`).

It is possible for the parser to produce overlapping tokens from the same piece of text. As an example, a hyphenated word will be reported both as the entire word and as each component:

    SELECT alias, description, token FROM ts_debug('foo-bar-beta1');
          alias      |               description                |     token
    -----------------+------------------------------------------+---------------
     numhword        | Hyphenated word, letters and digits      | foo-bar-beta1
     hword_asciipart | Hyphenated word part, all ASCII          | foo
     blank           | Space symbols                            | -
     hword_asciipart | Hyphenated word part, all ASCII          | bar
     blank           | Space symbols                            | -
     hword_numpart   | Hyphenated word part, letters and digits | beta1

This behavior is desirable since it allows searches to work for both the whole compound word and for components. Here is another instructive example:

    SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.html');
      alias   |  description  |            token
    ----------+---------------+------------------------------
     protocol | Protocol head | http://
     url      | URL           | example.com/stuff/index.html
     host     | Host          | example.com
     url_path | URL path      | /stuff/index.html

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