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
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.
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