variants and customization options.

</para>

</sect2>

<sect2 id="icu-locales">

<title>ICU Locales</title>

<sect3 id="icu-locale-names">

<title>ICU Locale Names</title>

<para>

The ICU format for the locale name is a <link

linkend="icu-language-tag">Language Tag</link>.

linkend="icu-language-tag">language tag</link> instead of relying on the

transformation.

</para>

<para>

A locale with no language name, or the special language name

<literal>root</literal>, is transformed to have the language

<literal>und</literal> ("undefined").

</para>

<para>

ICU can transform most libc locale names, as well as some other formats,

into language tags for easier transition to ICU. If a libc locale name is

used in ICU, it may not have precisely the same behavior as in libc.

</para>

<para>

If there is a problem interpreting the locale name, or if the locale name

represents a language or region that ICU does not recognize, you will see

<sect3 id="icu-language-tag">

<title>Language Tag</title>

<para>

A language tag, defined in BCP 47, is a standardized identifier used to

identify languages, regions, and other information about a locale.

</para>

<para>

Basic language tags are simply

<replaceable>language</replaceable><literal>-</literal><replaceable>region</replaceable>;

<literal>ja-JP</literal>, <literal>de</literal>, or

<literal>fr-CA</literal>.

</para>

<para>

Collation settings may be included in the language tag to customize

collation behavior. ICU allows extensive customization, such as

sensitivity (or insensitivity) to accents, case, and punctuation;

treatment of digits within text; and many other options to satisfy a

variety of uses.

</para>

<para>

To include this additional collation information in a language tag,

append <literal>-u</literal>, which indicates there are additional

<literal>-</literal><replaceable>value</replaceable>, which implies a

value of <literal>true</literal>.

</para>

<para>

For example, the language tag <literal>en-US-u-kn-ks-level2</literal>

means the locale with the English language in the US region, with

(1 row)

</screen>

</para>

<para>

See <xref linkend="icu-custom-collations"/> for details and additional

examples of using language tags with custom collation information for the

locale.

</para>

</sect3>

</sect2>

<sect2 id="locale-problems">

<title>Problems</title>

</tip>

</sect3>

</sect2>

<sect2 id="icu-custom-collations">

<title>ICU Custom Collations</title>

linkend="icu-collation-settings"/>, or see <xref

linkend="icu-external-references"/> for more details.

</para>

<sect3 id="icu-collation-comparison-levels">

<title>ICU Comparison Levels</title>

<para>

Comparison of two strings (collation) in ICU is determined by a

multi-level process, where textual features are grouped into

"levels". Treatment of each level is controlled by the <link

linkend="icu-collation-settings-table">collation settings</link>. Higher

levels correspond to finer textual features.

</para>

<para>

<xref linkend="icu-collation-levels"/> shows which textual feature

differences are considered significant when determining equality at the

given level. The unicode character <literal>U+2063</literal> is an

invisible separator, and as seen in the table, is ignored for at all

levels of comparison less than <literal>identic</literal>.

</para>

<table id="icu-collation-levels">

<title>ICU Collation Levels</title>

<tgroup cols="8">

<colspec colname="col6" colwidth="1*"/>

<colspec colname="col7" colwidth="1*"/>

<colspec colname="col8" colwidth="1*"/>

<thead>

<row>

<entry>Level</entry>

<entry><literal>'y' = 'z'</literal></entry>

</row>

</thead>

<tbody>

<row>

<entry>level1</entry>

</tgroup>

</table>

<para>

At every level, even with full normalization off, basic normalization is

performed. For example, <literal>'á'</literal> may be composed of the

code points <literal>U&amp;'\0061\0301'</literal> or the single code

created with <symbol>deterministic</symbol> set to

<literal>true</literal>.

</para>

<sect4 id="icu-collation-level-examples">

<title>Collation Level Examples</title>

<para>

<programlisting>

CREATE COLLATION level3 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level3');

SELECT 'x-y' = 'x_y' COLLATE level4; -- false

</programlisting>

</para>

</sect4>

</sect3>

<sect3 id="icu-collation-settings">

<title>Collation Settings for an ICU Locale</title>

<para>

<xref linkend="icu-collation-settings-table"/> shows the available

collation settings, which can be used as part of a language tag to

customize a collation.

</para>

<table id="icu-collation-settings-table">

<title>ICU Collation Settings</title>

<tgroup cols="4">

<colspec colname="col1" colwidth="1*"/>

<colspec colname="col2" colwidth="2*"/>

<colspec colname="col3" colwidth="2*"/>

<colspec colname="col4" colwidth="5*"/>

<thead>

<row>

<entry>Key</entry>

<entry>Description</entry>

</row>

</thead>

<tbody>

<row>

<entry><literal>co</literal></entry>

Collation type. See <xref linkend="icu-external-references"/> for additional options and details.

</entry>

</row>

<row>

<entry><literal>ka</literal></entry>

<entry><literal>noignore</literal>, <literal>shifted</literal></entry>

character classes are ignored.

</entry>

</row>

<row>

<entry><literal>kb</literal></entry>

<entry><literal>true</literal>, <literal>false</literal></entry>

before <literal>'aé'</literal>.

</entry>

</row>

<row>

<entry><literal>kc</literal></entry>

<entry><literal>true</literal>, <literal>false</literal></entry>

</para>

</entry>

</row>

<row>

<entry><literal>kf</literal></entry>

<entry>

the rules of the locale.

</entry>

</row>

<row>

<entry><literal>kn</literal></entry>

<entry><literal>true</literal>, <literal>false</literal></entry>

<literal>'id-123'</literal>.

</entry>

</row>

<row>

<entry><literal>kk</literal></entry>

<entry><literal>true</literal>, <literal>false</literal></entry>

</para>

</entry>

</row>

<row>

<entry><literal>kr</literal></entry>

<entry>

</para>

</entry>

</row>

<row>

<entry><literal>ks</literal></entry>

<entry><literal>level1</literal>, <literal>level2</literal>, <literal>level3</literal>, <literal>level4</literal>, <literal>identic</literal></entry>

<xref linkend="icu-collation-levels"/> for details.

</entry>

</row>

<row>

<entry><literal>kv</literal></entry>

<entry>

</tbody>

</tgroup>

</table>

Defaults may depend on locale. The above table is not meant to be

complete. See <xref linkend="icu-external-references"/> for additional

options and details.

<para>

Defaults may depend on locale. The above table is not meant to be

complete. See <xref linkend="icu-external-references"/> for additional

options and details.

</para>

<note>

<para>

For many collation settings, you must create the collation with

<sect3 id="icu-locale-examples">

<title>Examples</title>

<variablelist>

<varlistentry id="collation-managing-create-icu-de-u-co-phonebk-x-icu">

<term><literal>CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de-u-co-phonebk');</literal></term>

</listitem>

</varlistentry>

</variablelist>

</para>

</sect3>

<sect3 id="icu-external-references">

<title>External References for ICU</title>

<para>

This section (<xref linkend="icu-custom-collations"/>) is only a brief

overview of ICU behavior and language tags. Refer to the following

documents for technical details, additional options, and new behavior:

</para>

<itemizedlist>

<listitem>

<para>

<ulink

url="https://www.unicode.org/reports/tr35/tr35-collation.html">Unicode

Technical Standard #35</ulink>

<ulink url="https://www.unicode.org/reports/tr35/tr35-collation.html">Unicode Technical Standard #35</ulink>

</para>

</listitem>

<listitem>

</listitem>

<listitem>

<para>

<ulink url="https://github.com/unicode-org/cldr/blob/master/common/bcp47/collation.xml">CLDR

repository</ulink>

<ulink url="https://github.com/unicode-org/cldr/blob/master/common/bcp47/collation.xml">CLDR repository</ulink>

</para>

</listitem>

<listitem>