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.
This module is considered “trusted”, that is, it can be
installed by non-superusers who have CREATE privilege
on the current database.
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.
Some characters, like numeric symbols, may require whitespaces in their translation rule. It is possible to use double quotes around the translated characters in this case. A double quote needs to be escaped with a second double quote when including one in the translated character. For example:
¼ " 1/4" ½ " 1/2" ¾ " 3/4" “ """" ” """"
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).
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');
ts_lexize
-----------
{Hotel}
(1 row)
Here is an example showing how to insert the
unaccent dictionary into a text search configuration:
mydb=# CREATE TEXT SEARCH CONFIGURATION fr ( COPY = french );
mydb=# ALTER TEXT SEARCH CONFIGURATION fr
ALTER MAPPING FOR hword, hword_part, word
WITH unaccent, french_stem;
mydb=# select to_tsvector('fr','Hôtels de la Mer');
to_tsvector
-------------------
'hotel':1 'mer':4
(1 row)
mydb=# select to_tsvector('fr','Hôtel de la Mer') @@ to_tsquery('fr','Hotels');
?column?
----------
t
(1 row)
mydb=# select ts_headline('fr','Hôtel de la Mer',to_tsquery('fr','Hotels'));
ts_headline
------------------------
<b>Hôtel</b> de la Mer
(1 row)
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([dictionaryregdictionary, ]stringtext) returnstext
If the dictionary argument is
omitted, the text search dictionary named unaccent and
appearing in the same schema as the unaccent()
function itself is used.
For example:
SELECT unaccent('unaccent', 'Hôtel');
SELECT unaccent('Hôtel');