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F.44. unaccent

unaccent is a text search dictionary that removes accents (diacritic signs) from lexemes. It's a filtering dictionary, which means its output is always passed to the next dictionary (if any), unlike the normal behavior of dictionaries. This allows accent-insensitive processing for full text search.

The current implementation of unaccent cannot be used as a normalizing dictionary for the thesaurus dictionary.

F.44.1. Configuration

An unaccent dictionary accepts the following options:

  • RULES is the base name of the file containing the list of translation rules. This file must be stored in $SHAREDIR/tsearch_data/ (where $SHAREDIR means the PostgreSQL installation's shared-data directory). Its name must end in .rules (which is not to be included in the RULES parameter).

The rules file has the following format:

  • Each line represents one translation rule, consisting of a character with accent followed by a character without accent. The first is translated into the second. For example,

    À        A
    Á        A
    Â        A
    Ã        A
    Ä        A
    Å        A
    Æ        AE

    The two characters must be separated by whitespace, and any leading or trailing whitespace on a line is ignored.

  • Alternatively, if only one character is given on a line, instances of that character are deleted; this is useful in languages where accents are represented by separate characters.

  • Actually, each "character" can be any string not containing whitespace, so unaccent dictionaries could be used for other sorts of substring substitutions besides diacritic removal.

  • As with other PostgreSQL text search configuration files, the rules file must be stored in UTF-8 encoding. The data is automatically translated into the current database's encoding when loaded. Any lines containing untranslatable characters are silently ignored, so that rules files can contain rules that are not applicable in the current encoding.

A more complete example, which is directly useful for most European languages, can be found in unaccent.rules, which is installed in $SHAREDIR/tsearch_data/ when the unaccent module is installed. This rules file translates characters with accents to the same characters without accents, and it also expands ligatures into the equivalent series of simple characters (for example, Æ to AE).

F.44.2. Usage

Installing the unaccent extension creates a text search template unaccent and a dictionary unaccent based on it. The unaccent dictionary has the default parameter setting RULES='unaccent', which makes it immediately usable with the standard unaccent.rules file. If you wish, you can alter the parameter, for example

mydb=# ALTER TEXT SEARCH DICTIONARY unaccent (RULES='my_rules');

or create new dictionaries based on the template.

To test the dictionary, you can try:

mydb=# select ts_lexize('unaccent','Hôtel');
(1 row)

Here is an example showing how to insert the unaccent dictionary into a text search configuration:

        ALTER MAPPING FOR hword, hword_part, word
        WITH unaccent, french_stem;
mydb=# select to_tsvector('fr','Hôtels de la Mer');
 'hotel':1 'mer':4
(1 row)

mydb=# select to_tsvector('fr','Hôtel de la Mer') @@ to_tsquery('fr','Hotels');
(1 row)

mydb=# select ts_headline('fr','Hôtel de la Mer',to_tsquery('fr','Hotels'));
 <b>Hôtel</b> de la Mer
(1 row)

F.44.3. Functions

The unaccent() function removes accents (diacritic signs) from a given string. Basically, it's a wrapper around unaccent-type dictionaries, but it can be used outside normal text search contexts.

unaccent([dictionary, ] string) returns text

If the dictionary argument is omitted, unaccent is assumed.

For example:

SELECT unaccent('unaccent', 'Hôtel');
SELECT unaccent('Hôtel');

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