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 entity XML entity & 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 XML declarations (but note that this includes anything starting 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.h tml'); 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