# Localization

This chapter describes the available localization features from the point of view of the administrator. PostgreSQL supports two localization facilities:

- Using the locale features of the operating system to provide locale-specific collation order, number formatting, translated messages, and other aspects. This is covered in locale and collation.

- Providing a number of different character sets to support storing text in all kinds of languages, and providing character set translation between client and server. This is covered in multibyte.

## Locale Support

Locale support refers to an application respecting cultural preferences regarding alphabets, sorting, number formatting, etc. PostgreSQL uses the standard ISO C and POSIX locale facilities provided by the server operating system. For additional information refer to the documentation of your system.

### Overview

Locale support is automatically initialized when a database cluster is created using `initdb`. `initdb` will initialize the database cluster with the locale setting of its execution environment by default, so if your system is already set to use the locale that you want in your database cluster then there is nothing else you need to do. If you want to use a different locale (or you are not sure which locale your system is set to), you can instruct `initdb` exactly which locale to use by specifying the `--locale` option. For example:

    initdb --locale=sv_SE

This example for Unix systems sets the locale to Swedish (`sv`) as spoken in Sweden (`SE`). Other possibilities might include `en_US` (U.S. English) and `fr_CA` (French Canadian). If more than one character set can be used for a locale then the specifications can take the form `language_territory.codeset`. For example, `fr_BE.UTF-8` represents the French language (fr) as spoken in Belgium (BE), with a UTF-8 character set encoding.

What locales are available on your system under what names depends on what was provided by the operating system vendor and what was installed. On most Unix systems, the command `locale -a` will provide a list of available locales. Windows uses more verbose locale names, such as `German_Germany` or `Swedish_Sweden.1252`, but the principles are the same.

Occasionally it is useful to mix rules from several locales, e.g., use English collation rules but Spanish messages. To support that, a set of locale subcategories exist that control only certain aspects of the localization rules:

`LC_COLLATE` | String sort order
`LC_CTYPE` | Character classification (What is a letter? Its upper-case equivalent?)
`LC_MESSAGES` | Language of messages
`LC_MONETARY` | Formatting of currency amounts
`LC_NUMERIC` | Formatting of numbers
`LC_TIME` | Formatting of dates and times

The category names translate into names of `initdb` options to override the locale choice for a specific category. For instance, to set the locale to French Canadian, but use U.S. rules for formatting currency, use `initdb --locale=fr_CA --lc-monetary=en_US`.

If you want the system to behave as if it had no locale support, use the special locale name `C`, or equivalently `POSIX`.

Some locale categories must have their values fixed when the database is created. You can use different settings for different databases, but once a database is created, you cannot change them for that database anymore. `LC_COLLATE` and `LC_CTYPE` are these categories. They affect the sort order of indexes, so they must be kept fixed, or indexes on text columns would become corrupt. (But you can alleviate this restriction using collations, as discussed in collation.) The default values for these categories are determined when `initdb` is run, and those values are used when new databases are created, unless specified otherwise in the `CREATE DATABASE` command.

The other locale categories can be changed whenever desired by setting the server configuration parameters that have the same name as the locale categories (see runtime-config-client-format for details). The values that are chosen by `initdb` are actually only written into the configuration file `postgresql.conf` to serve as defaults when the server is started. If you remove these assignments from `postgresql.conf` then the server will inherit the settings from its execution environment.

Note that the locale behavior of the server is determined by the environment variables seen by the server, not by the environment of any client. Therefore, be careful to configure the correct locale settings before starting the server. A consequence of this is that if client and server are set up in different locales, messages might appear in different languages depending on where they originated.

When we speak of inheriting the locale from the execution environment, this means the following on most operating systems: For a given locale category, say the collation, the following environment variables are consulted in this order until one is found to be set: `LC_ALL`, `LC_COLLATE` (or the variable corresponding to the respective category), `LANG`. If none of these environment variables are set then the locale defaults to `C`.

Some message localization libraries also look at the environment variable `LANGUAGE` which overrides all other locale settings for the purpose of setting the language of messages. If in doubt, please refer to the documentation of your operating system, in particular the documentation about gettext.

To enable messages to be translated to the user's preferred language, NLS must have been selected at build time (`configure --enable-nls`). All other locale support is built in automatically.

### Behavior

The locale settings influence the following SQL features:

- Sort order in queries using `ORDER BY` or the standard comparison operators on textual data

- The `upper`, `lower`, and `initcap` functions

- Pattern matching operators (`LIKE`, `SIMILAR TO`, and POSIX-style regular expressions); locales affect both case insensitive matching and the classification of characters by character-class regular expressions

- The `to_char` family of functions

- The ability to use indexes with `LIKE` clauses

The drawback of using locales other than `C` or `POSIX` in PostgreSQL is its performance impact. It slows character handling and prevents ordinary indexes from being used by `LIKE`. For this reason use locales only if you actually need them.

As a workaround to allow PostgreSQL to use indexes with `LIKE` clauses under a non-C locale, several custom operator classes exist. These allow the creation of an index that performs a strict character-by-character comparison, ignoring locale comparison rules. Refer to indexes-opclass for more information. Another approach is to create indexes using the `C` collation, as discussed in collation.

### Selecting Locales

Locales can be selected in different scopes depending on requirements. The above overview showed how locales are specified using `initdb` to set the defaults for the entire cluster. The following list shows where locales can be selected. Each item provides the defaults for the subsequent items, and each lower item allows overriding the defaults on a finer granularity.

1. As explained above, the environment of the operating system provides the defaults for the locales of a newly initialized database cluster. In many cases, this is enough: if the operating system is configured for the desired language/territory, by default PostgreSQL will also behave according to that locale.

2. As shown above, command-line options for `initdb` specify the locale settings for a newly initialized database cluster. Use this if the operating system does not have the locale configuration you want for your database system.

3. A locale can be selected separately for each database. The SQL command `CREATE DATABASE` and its command-line equivalent `createdb` have options for that. Use this for example if a database cluster houses databases for multiple tenants with different requirements.

4. Locale settings can be made for individual table columns. This uses an SQL object called collation and is explained in collation. Use this for example to sort data in different languages or customize the sort order of a particular table.

5. Finally, locales can be selected for an individual query. Again, this uses SQL collation objects. This could be used to change the sort order based on run-time choices or for ad-hoc experimentation.

### Locale Providers

A locale provider specifies which library defines the locale behavior for collations and character classifications.

The commands and tools that select the locale settings, as described above, each have an option to select the locale provider. Here is an example to initialize a database cluster using the ICU provider:

    initdb --locale-provider=icu --icu-locale=en

See the description of the respective commands and programs for details. Note that you can mix locale providers at different granularities, for example use `libc` by default for the cluster but have one database that uses the `icu` provider, and then have collation objects using either provider within those databases.

Regardless of the locale provider, the operating system is still used to provide some locale-aware behavior, such as messages (see guc-lc-messages).

The available locale providers are listed below:

`builtin`

- The `builtin` provider uses built-in operations. Only the `C` and `C.UTF-8` locales are supported for this provider.

  The `C` locale behavior is identical to the `C` locale in the libc provider. When using this locale, the behavior may depend on the database encoding.

  The `C.UTF-8` locale is available only for when the database encoding is `UTF-8`, and the behavior is based on Unicode. The collation uses the code point values only. The regular expression character classes are based on the "POSIX Compatible" semantics, and the case mapping is the "simple" variant.

`icu`

- The `icu` provider uses the external ICU
  library. PostgreSQL must have been configured with support.

  ICU provides collation and character classification behavior that is independent of the operating system and database encoding, which is preferable if you expect to transition to other platforms without any change in results. `LC_COLLATE` and `LC_CTYPE` can be set independently of the ICU locale.

  For the ICU provider, results may depend on the version of the ICU library used, as it is updated to reflect changes in natural language over time.

`libc`

- The `libc` provider uses the operating system's C library. The collation and character classification behavior is controlled by the settings `LC_COLLATE` and `LC_CTYPE`, so they cannot be set independently.

  The same locale name may have different behavior on different platforms when using the libc provider.

### ICU Locales

#### ICU Locale Names

The ICU format for the locale name is a Language Tag.

    CREATE COLLATION mycollation1 (provider = icu, locale = 'ja-JP');
    CREATE COLLATION mycollation2 (provider = icu, locale = 'fr');

#### Locale Canonicalization and Validation

When defining a new ICU collation object or database with ICU as the provider, the given locale name is transformed ("canonicalized") into a language tag if not already in that form. For instance,

    CREATE COLLATION mycollation3 (provider = icu, locale = 'en-US-u-kn-true');
    NOTICE:  using standard form "en-US-u-kn" for locale "en-US-u-kn-true"
    CREATE COLLATION mycollation4 (provider = icu, locale = 'de_DE.utf8');
    NOTICE:  using standard form "de-DE" for locale "de_DE.utf8"

If you see this notice, ensure that the provider and locale are the expected result. For consistent results when using the ICU provider, specify the canonical language tag instead of relying on the transformation.

A locale with no language name, or the special language name `root`, is transformed to have the language `und` ("undefined").

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.

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 the following warning:

    CREATE COLLATION nonsense (provider = icu, locale = 'nonsense');
    WARNING:  ICU locale "nonsense" has unknown language "nonsense"
    HINT:  To disable ICU locale validation, set parameter icu_validation_level to DISABLED.
    CREATE COLLATION

guc-icu-validation-level controls how the message is reported. Unless set to `ERROR`, the collation will still be created, but the behavior may not be what the user intended.

#### Language Tag

A language tag, defined in BCP 47, is a standardized identifier used to identify languages, regions, and other information about a locale.

Basic language tags are simply `language``-``region`; or even just `language`. The `language` is a language code (e.g. `fr` for French), and `region` is a region code (e.g. `CA` for Canada). Examples: `ja-JP`, `de`, or `fr-CA`.

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.

To include this additional collation information in a language tag, append `-u`, which indicates there are additional collation settings, followed by one or more `-``key``-``value` pairs. The `key` is the key for a collation setting and `value` is a valid value for that setting. For boolean settings, the `-``key` may be specified without a corresponding `-``value`, which implies a value of `true`.

For example, the language tag `en-US-u-kn-ks-level2` means the locale with the English language in the US region, with collation settings `kn` set to `true` and `ks` set to `level2`. Those settings mean the collation will be case-insensitive and treat a sequence of digits as a single number:

    CREATE COLLATION mycollation5 (provider = icu, deterministic = false, locale = 'en-US-u-kn-ks-level2');
    SELECT 'aB' = 'Ab' COLLATE mycollation5 as result;
     result
    --------
     t
    (1 row)

    SELECT 'N-45' < 'N-123' COLLATE mycollation5 as result;
     result
    --------
     t
    (1 row)

See icu-custom-collations for details and additional examples of using language tags with custom collation information for the locale.

### Problems

If locale support doesn't work according to the explanation above, check that the locale support in your operating system is correctly configured. To check what locales are installed on your system, you can use the command `locale -a` if your operating system provides it.

Check that PostgreSQL is actually using the locale that you think it is. The `LC_COLLATE` and `LC_CTYPE` settings are determined when a database is created, and cannot be changed except by creating a new database. Other locale settings including `LC_MESSAGES` and `LC_MONETARY` are initially determined by the environment the server is started in, but can be changed on-the-fly. You can check the active locale settings using the `SHOW` command.

The directory `src/test/locale` in the source distribution contains a test suite for PostgreSQL's locale support.

Client applications that handle server-side errors by parsing the text of the error message will obviously have problems when the server's messages are in a different language. Authors of such applications are advised to make use of the error code scheme instead.

Maintaining catalogs of message translations requires the on-going efforts of many volunteers that want to see PostgreSQL speak their preferred language well. If messages in your language are currently not available or not fully translated, your assistance would be appreciated. If you want to help, refer to nls or write to the developers' mailing list.

## Collation Support

The collation feature allows specifying the sort order and character classification behavior of data per-column, or even per-operation. This alleviates the restriction that the LC_COLLATE and LC_CTYPE settings of a database cannot be changed after its creation.

### Concepts

Conceptually, every expression of a collatable data type has a collation. (The built-in collatable data types are `text`, `varchar`, and `char`. User-defined base types can also be marked collatable, and of course a domain over a collatable data type is collatable.) If the expression is a column reference, the collation of the expression is the defined collation of the column. If the expression is a constant, the collation is the default collation of the data type of the constant. The collation of a more complex expression is derived from the collations of its inputs, as described below.

The collation of an expression can be the default collation, which means the locale settings defined for the database. It is also possible for an expression's collation to be indeterminate. In such cases, ordering operations and other operations that need to know the collation will fail.

When the database system has to perform an ordering or a character classification, it uses the collation of the input expression. This happens, for example, with `ORDER BY` clauses and function or operator calls such as `<`. The collation to apply for an `ORDER BY` clause is simply the collation of the sort key. The collation to apply for a function or operator call is derived from the arguments, as described below. In addition to comparison operators, collations are taken into account by functions that convert between lower and upper case letters, such as `lower`, `upper`, and `initcap`; by pattern matching operators; and by `to_char` and related functions.

For a function or operator call, the collation that is derived by examining the argument collations is used at run time for performing the specified operation. If the result of the function or operator call is of a collatable data type, the collation is also used at parse time as the defined collation of the function or operator expression, in case there is a surrounding expression that requires knowledge of its collation.

The collation derivation of an expression can be implicit or explicit. This distinction affects how collations are combined when multiple different collations appear in an expression. An explicit collation derivation occurs when a `COLLATE` clause is used; all other collation derivations are implicit. When multiple collations need to be combined, for example in a function call, the following rules are used:

1. If any input expression has an explicit collation derivation, then all explicitly derived collations among the input expressions must be the same, otherwise an error is raised. If any explicitly derived collation is present, that is the result of the collation combination.

2. Otherwise, all input expressions must have the same implicit collation derivation or the default collation. If any non-default collation is present, that is the result of the collation combination. Otherwise, the result is the default collation.

3. If there are conflicting non-default implicit collations among the input expressions, then the combination is deemed to have indeterminate collation. This is not an error condition unless the particular function being invoked requires knowledge of the collation it should apply. If it does, an error will be raised at run-time.

For example, consider this table definition:

    CREATE TABLE test1 (
        a text COLLATE "de_DE",
        b text COLLATE "es_ES",
        ...
    );

Then in

    SELECT a < 'foo' FROM test1;

the `<` comparison is performed according to `de_DE` rules, because the expression combines an implicitly derived collation with the default collation. But in

    SELECT a < ('foo' COLLATE "fr_FR") FROM test1;

the comparison is performed using `fr_FR` rules, because the explicit collation derivation overrides the implicit one. Furthermore, given

    SELECT a < b FROM test1;

the parser cannot determine which collation to apply, since the a and b columns have conflicting implicit collations. Since the `<` operator does need to know which collation to use, this will result in an error. The error can be resolved by attaching an explicit collation specifier to either input expression, thus:

    SELECT a < b COLLATE "de_DE" FROM test1;

or equivalently

    SELECT a COLLATE "de_DE" < b FROM test1;

On the other hand, the structurally similar case

    SELECT a || b FROM test1;

does not result in an error, because the `||` operator does not care about collations: its result is the same regardless of the collation.

The collation assigned to a function or operator's combined input expressions is also considered to apply to the function or operator's result, if the function or operator delivers a result of a collatable data type. So, in

    SELECT * FROM test1 ORDER BY a || 'foo';

the ordering will be done according to `de_DE` rules. But this query:

    SELECT * FROM test1 ORDER BY a || b;

results in an error, because even though the `||` operator doesn't need to know a collation, the `ORDER BY` clause does. As before, the conflict can be resolved with an explicit collation specifier:

    SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";

### Managing Collations

A collation is an SQL schema object that maps an SQL name to locales provided by libraries installed in the operating system. A collation definition has a provider that specifies which library supplies the locale data. One standard provider name is `libc`, which uses the locales provided by the operating system C library. These are the locales used by most tools provided by the operating system. Another provider is `icu`, which uses the external ICU
library. ICU locales can only be used if support for ICU was configured when PostgreSQL was built.

A collation object provided by `libc` maps to a combination of LC_COLLATE and LC_CTYPE settings, as accepted by the `setlocale()` system library call. (As the name would suggest, the main purpose of a collation is to set LC_COLLATE, which controls the sort order. But it is rarely necessary in practice to have an LC_CTYPE setting that is different from LC_COLLATE, so it is more convenient to collect these under one concept than to create another infrastructure for setting LC_CTYPE per expression.) Also, a `libc` collation is tied to a character set encoding (see multibyte). The same collation name may exist for different encodings.

A collation object provided by `icu` maps to a named collator provided by the ICU library. ICU does not support separate collate and ctype settings, so they are always the same. Also, ICU collations are independent of the encoding, so there is always only one ICU collation of a given name in a database.

#### Standard Collations

On all platforms, the following collations are supported:

`unicode`

- This SQL standard collation sorts using the Unicode Collation Algorithm with the Default Unicode Collation Element Table. It is available in all encodings. ICU support is required to use this collation, and behavior may change if Postgres is built with a different version of ICU. (This collation has the same behavior as the ICU root locale; see collation-managing-predefined-icu-und-x-icu.)

`ucs_basic`

- This SQL standard collation sorts using the Unicode code point values rather than natural language order, and only the ASCII letters `A` through `Z` are treated as letters. The behavior is efficient and stable across all versions. Only available for encoding `UTF8`. (This collation has the same behavior as the libc locale specification `C` in `UTF8` encoding.)

`pg_c_utf8`

- This collation sorts by Unicode code point values rather than natural language order. For the functions `lower`, `initcap`, and `upper`, it uses Unicode simple case mapping. For pattern matching (including regular expressions), it uses the POSIX Compatible variant of Unicode Compatibility Properties. Behavior is efficient and stable within a Postgres major version. This collation is only available for encoding `UTF8`.

`C` (equivalent to `POSIX`)

- The `C` and `POSIX` collations are based on traditional C behavior. They sort by byte values rather than natural language order, and only the ASCII letters `A` through `Z` are treated as letters. The behavior is efficient and stable across all versions for a given database encoding, but behavior may vary between different database encodings.

`default`

- The `default` collation selects the locale specified at database creation time.

Additional collations may be available depending on operating system support. The efficiency and stability of these additional collations depend on the collation provider, the provider version, and the locale.

#### Predefined Collations

If the operating system provides support for using multiple locales within a single program (`newlocale` and related functions), or if support for ICU is configured, then when a database cluster is initialized, `initdb` populates the system catalog `pg_collation` with collations based on all the locales it finds in the operating system at the time.

To inspect the currently available locales, use the query `SELECT * FROM pg_collation`, or the command `\dOS+` in psql.

##### libc Collations

For example, the operating system might provide a locale named `de_DE.utf8`. `initdb` would then create a collation named `de_DE.utf8` for encoding `UTF8` that has both LC_COLLATE and LC_CTYPE set to `de_DE.utf8`. It will also create a collation with the `.utf8` tag stripped off the name. So you could also use the collation under the name `de_DE`, which is less cumbersome to write and makes the name less encoding-dependent. Note that, nevertheless, the initial set of collation names is platform-dependent.

The default set of collations provided by `libc` map directly to the locales installed in the operating system, which can be listed using the command `locale -a`. In case a `libc` collation is needed that has different values for LC_COLLATE and LC_CTYPE, or if new locales are installed in the operating system after the database system was initialized, then a new collation may be created using the sql-createcollation command. New operating system locales can also be imported en masse using the `pg_import_system_collations()` function.

Within any particular database, only collations that use that database's encoding are of interest. Other entries in `pg_collation` are ignored. Thus, a stripped collation name such as `de_DE` can be considered unique within a given database even though it would not be unique globally. Use of the stripped collation names is recommended, since it will make one fewer thing you need to change if you decide to change to another database encoding. Note however that the `default`, `C`, and `POSIX` collations can be used regardless of the database encoding.

PostgreSQL considers distinct collation objects to be incompatible even when they have identical properties. Thus for example,

    SELECT a COLLATE "C" < b COLLATE "POSIX" FROM test1;

will draw an error even though the `C` and `POSIX` collations have identical behaviors. Mixing stripped and non-stripped collation names is therefore not recommended.

##### ICU Collations

With ICU, it is not sensible to enumerate all possible locale names. ICU uses a particular naming system for locales, but there are many more ways to name a locale than there are actually distinct locales. `initdb` uses the ICU APIs to extract a set of distinct locales to populate the initial set of collations. Collations provided by ICU are created in the SQL environment with names in BCP 47 language tag format, with a private use extension `-x-icu` appended, to distinguish them from libc locales.

Here are some example collations that might be created:

`de-x-icu`

- German collation, default variant

`de-AT-x-icu`

- German collation for Austria, default variant

  (There are also, say, `de-DE-x-icu` or `de-CH-x-icu`, but as of this writing, they are equivalent to `de-x-icu`.)

`und-x-icu` (for undefined)

- ICU root collation. Use this to get a reasonable language-agnostic sort order.

Some (less frequently used) encodings are not supported by ICU. When the database encoding is one of these, ICU collation entries in `pg_collation` are ignored. Attempting to use one will draw an error along the lines of collation "de-x-icu" for encoding "WIN874" does not exist.

#### Creating New Collation Objects

If the standard and predefined collations are not sufficient, users can create their own collation objects using the SQL command sql-createcollation.

The standard and predefined collations are in the schema `pg_catalog`, like all predefined objects. User-defined collations should be created in user schemas. This also ensures that they are saved by `pg_dump`.

##### libc Collations

New libc collations can be created like this:

    CREATE COLLATION german (provider = libc, locale = 'de_DE');

The exact values that are acceptable for the `locale` clause in this command depend on the operating system. On Unix-like systems, the command `locale -a` will show a list.

Since the predefined libc collations already include all collations defined in the operating system when the database instance is initialized, it is not often necessary to manually create new ones. Reasons might be if a different naming system is desired (in which case see also collation-copy) or if the operating system has been upgraded to provide new locale definitions (in which case see also `pg_import_system_collations()`).

##### ICU Collations

ICU collations can be created like:

    CREATE COLLATION german (provider = icu, locale = 'de-DE');

ICU locales are specified as a BCP 47 Language Tag, but can also accept most libc-style locale names. If possible, libc-style locale names are transformed into language tags.

New ICU collations can customize collation behavior extensively by including collation attributes in the language tag. See icu-custom-collations for details and examples.

##### Copying Collations

The command sql-createcollation can also be used to create a new collation from an existing collation, which can be useful to be able to use operating-system-independent collation names in applications, create compatibility names, or use an ICU-provided collation under a more readable name. For example:

    CREATE COLLATION german FROM "de_DE";
    CREATE COLLATION french FROM "fr-x-icu";

#### Nondeterministic Collations

A collation is either deterministic or nondeterministic. A deterministic collation uses deterministic comparisons, which means that it considers strings to be equal only if they consist of the same byte sequence. Nondeterministic comparison may determine strings to be equal even if they consist of different bytes. Typical situations include case-insensitive comparison, accent-insensitive comparison, as well as comparison of strings in different Unicode normal forms. It is up to the collation provider to actually implement such insensitive comparisons; the deterministic flag only determines whether ties are to be broken using bytewise comparison. See also Unicode Technical Standard 10 for more information on the terminology.

To create a nondeterministic collation, specify the property `deterministic = false` to `CREATE COLLATION`, for example:

    CREATE COLLATION ndcoll (provider = icu, locale = 'und', deterministic = false);

This example would use the standard Unicode collation in a nondeterministic way. In particular, this would allow strings in different normal forms to be compared correctly. More interesting examples make use of the ICU customization facilities explained above. For example:

    CREATE COLLATION case_insensitive (provider = icu, locale = 'und-u-ks-level2', deterministic = false);
    CREATE COLLATION ignore_accents (provider = icu, locale = 'und-u-ks-level1-kc-true', deterministic = false);

All standard and predefined collations are deterministic, all user-defined collations are deterministic by default. While nondeterministic collations give a more correct behavior, especially when considering the full power of Unicode and its many special cases, they also have some drawbacks. Foremost, their use leads to a performance penalty. Note, in particular, that B-tree cannot use deduplication with indexes that use a nondeterministic collation. Also, certain operations are not possible with nondeterministic collations, such as pattern matching operations. Therefore, they should be used only in cases where they are specifically wanted.

To deal with text in different Unicode normalization forms, it is also an option to use the functions/expressions `normalize` and `is normalized` to preprocess or check the strings, instead of using nondeterministic collations. There are different trade-offs for each approach.

### ICU Custom Collations

ICU allows extensive control over collation behavior by defining new collations with collation settings as a part of the language tag. These settings can modify the collation order to suit a variety of needs. For instance:

    -- ignore differences in accents and case
    CREATE COLLATION ignore_accent_case (provider = icu, deterministic = false, locale = 'und-u-ks-level1');
    SELECT 'Å' = 'A' COLLATE ignore_accent_case; -- true
    SELECT 'z' = 'Z' COLLATE ignore_accent_case; -- true

    -- upper case letters sort before lower case.
    CREATE COLLATION upper_first (provider = icu, locale = 'und-u-kf-upper');
    SELECT 'B' < 'b' COLLATE upper_first; -- true

    -- treat digits numerically and ignore punctuation
    CREATE COLLATION num_ignore_punct (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-kn');
    SELECT 'id-45' < 'id-123' COLLATE num_ignore_punct; -- true
    SELECT 'w;x*y-z' = 'wxyz' COLLATE num_ignore_punct; -- true

Many of the available options are described in icu-collation-settings, or see icu-external-references for more details.

#### ICU Comparison Levels

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 collation settings. Higher levels correspond to finer textual features.

icu-collation-levels shows which textual feature differences are considered significant when determining equality at the given level. The Unicode character `U+2063` is an invisible separator, and as seen in the table, is ignored for at all levels of comparison less than `identic`.

#### ICU Collation Levels

Level | Description | `'f' = 'f'` | `'ab' = U&'a\2063b'` | `'x-y' = 'x_y'` | `'g' = 'G'` | `'n' = 'ñ'` | `'y' = 'z'`
level1 | Base Character | `true` | `true` | `true` | `true` | `true` | `false`
level2 | Accents | `true` | `true` | `true` | `true` | `false` | `false`
level3 | Case/Variants | `true` | `true` | `true` | `false` | `false` | `false`
level4 | Punctuation only with `ka-shifted`; see icu-collation-settings-table | `true` | `true` | `false` | `false` | `false` | `false`
identic | All | `true` | `false` | `false` | `false` | `false` | `false`

At every level, even with full normalization off, basic normalization is performed. For example, `'á'` may be composed of the code points `U&'\0061\0301'` or the single code point `U&'\00E1'`, and those sequences will be considered equal even at the `identic` level. To treat any difference in code point representation as distinct, use a collation created with deterministic set to `true`.

##### Collation Level Examples

    CREATE COLLATION level3 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level3');
    CREATE COLLATION level4 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level4');
    CREATE COLLATION identic (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-identic');

    -- invisible separator ignored at all levels except identic
    SELECT 'ab' = U&'a\2063b' COLLATE level4; -- true
    SELECT 'ab' = U&'a\2063b' COLLATE identic; -- false

    -- punctuation ignored at level3 but not at level 4
    SELECT 'x-y' = 'x_y' COLLATE level3; -- true
    SELECT 'x-y' = 'x_y' COLLATE level4; -- false

#### Collation Settings for an ICU Locale

icu-collation-settings-table shows the available collation settings, which can be used as part of a language tag to customize a collation.

#### ICU Collation Settings

Key | Values | Default | Description
`co` | `emoji`, `phonebk`, `standard`, `...` | `standard` | Collation type. See icu-external-references for additional options and details.
`ka` | `noignore`, `shifted` | `noignore` | If set to `shifted`, causes some characters (e.g. punctuation or space) to be ignored in comparison. Key `ks` must be set to `level3` or lower to take effect. Set key `kv` to control which character classes are ignored.
`kb` | `true`, `false` | `false` | Backwards comparison for the level 2 differences. For example, locale `und-u-kb` sorts `'àe'` before `'aé'`.
`kc` | `true`, `false` | `false` | Separates case into a "level 2.5" that falls between accents and other level 3 features. If set to `true` and `ks` is set to `level1`, will ignore accents but take case into account.
`kf` | `upper`, `lower`, `false` | `false` | If set to `upper`, upper case sorts before lower case. If set to `lower`, lower case sorts before upper case. If set to `false`, the sort depends on the rules of the locale.
`kn` | `true`, `false` | `false` | If set to `true`, numbers within a string are treated as a single numeric value rather than a sequence of digits. For example, `'id-45'` sorts before `'id-123'`.
`kk` | `true`, `false` | `false` | Enable full normalization; may affect performance. Basic normalization is performed even when set to `false`. Locales for languages that require full normalization typically enable it by default. Full normalization is important in some cases, such as when multiple accents are applied to a single character. For example, the code point sequences `U&'\0065\0323\0302'` and `U&'\0065\0302\0323'` represent an `e` with circumflex and dot-below accents applied in different orders. With full normalization on, these code point sequences are treated as equal; otherwise they are unequal.
`kr` | `space`, `punct`, `symbol`, `currency`, `digit`, `script-id` |  | Set to one or more of the valid values, or any BCP 47 `script-id`, e.g. `latn` ("Latin") or `grek` ("Greek"). Multiple values are separated by "`-`". Redefines the ordering of classes of characters; those characters belonging to a class earlier in the list sort before characters belonging to a class later in the list. For instance, the value `digit-currency-space` (as part of a language tag like `und-u-kr-digit-currency-space`) sorts punctuation before digits and spaces.
`ks` | `level1`, `level2`, `level3`, `level4`, `identic` | `level3` | Sensitivity (or "strength") when determining equality, with `level1` the least sensitive to differences and `identic` the most sensitive to differences. See icu-collation-levels for details.
`kv` | `space`, `punct`, `symbol`, `currency` | `punct` | Classes of characters ignored during comparison at level 3. Setting to a later value includes earlier values; e.g. `symbol` also includes `punct` and `space` in the characters to be ignored. Key `ka` must be set to `shifted` and key `ks` must be set to `level3` or lower to take effect.

Defaults may depend on locale. The above table is not meant to be complete. See icu-external-references for additional options and details.

For many collation settings, you must create the collation with `deterministic` set to `false` for the setting to have the desired effect (see collation-nondeterministic). Additionally, some settings only take effect when the key `ka` is set to `shifted` (see icu-collation-settings-table).

#### Collation Settings Examples

`CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de-u-co-phonebk');`

- German collation with phone book collation type

`CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = 'und-u-co-emoji');`

- Root collation with Emoji collation type, per Unicode Technical Standard #51

`CREATE COLLATION latinlast (provider = icu, locale = 'en-u-kr-grek-latn');`

- Sort Greek letters before Latin ones. (The default is Latin before Greek.)

`CREATE COLLATION upperfirst (provider = icu, locale = 'en-u-kf-upper');`

- Sort upper-case letters before lower-case letters. (The default is lower-case letters first.)

`CREATE COLLATION special (provider = icu, locale = 'en-u-kf-upper-kr-grek-latn');`

- Combines both of the above options.

#### ICU Tailoring Rules

If the options provided by the collation settings shown above are not sufficient, the order of collation elements can be changed with tailoring rules, whose syntax is detailed at .

This small example creates a collation based on the root locale with a tailoring rule:

    CREATE COLLATION custom (provider = icu, locale = 'und', rules = '&V << w <<< W');

With this rule, the letter W is sorted after V, but is treated as a secondary difference similar to an accent. Rules like this are contained in the locale definitions of some languages. (Of course, if a locale definition already contains the desired rules, then they don't need to be specified again explicitly.)

Here is a more complex example. The following statement sets up a collation named `ebcdic` with rules to sort US-ASCII characters in the order of the EBCDIC encoding.

    CREATE COLLATION ebcdic (provider = icu, locale = 'und',
    rules = $$
    & ' ' < '.' < '<' < '(' < '+' < \|
    < '&' < '!' < '$' < '*' < ')' < ';'
    < '-' < '/' < ',' < '%' < '_' < '>' < '?'
    < '`' < ':' < '#' < '@' < \' < '=' < '"'
    <*a-r < '~' <*s-z < '^' < '[' < ']'
    < '{' <*A-I < '}' <*J-R < '\' <*S-Z <*0-9
    $$);

    SELECT c
    FROM (VALUES ('a'), ('b'), ('A'), ('B'), ('1'), ('2'), ('!'), ('^')) AS x(c)
    ORDER BY c COLLATE ebcdic;
     c
    ---
     !
     a
     b
     ^
     A
     B
     1
     2

#### External References for ICU

This section (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:

- Unicode Technical Standard #35

- BCP 47

- CLDR repository

## Character Set Support

The character set support in PostgreSQL allows you to store text in a variety of character sets (also called encodings), including single-byte character sets such as the ISO 8859 series and multiple-byte character sets such as EUC (Extended Unix Code), UTF-8, and Mule internal code. All supported character sets can be used transparently by clients, but a few are not supported for use within the server (that is, as a server-side encoding). The default character set is selected while initializing your PostgreSQL database cluster using `initdb`. It can be overridden when you create a database, so you can have multiple databases each with a different character set.

An important restriction, however, is that each database's character set must be compatible with the database's `LC_CTYPE` (character classification) and `LC_COLLATE` (string sort order) locale settings. For `C` or `POSIX` locale, any character set is allowed, but for other libc-provided locales there is only one character set that will work correctly. (On Windows, however, UTF-8 encoding can be used with any locale.) If you have ICU support configured, ICU-provided locales can be used with most but not all server-side encodings.

### Supported Character Sets

charset-table shows the character sets available for use in PostgreSQL.

### PostgreSQL Character Sets

Name | Description | Language | Server? | ICU? | Bytes/&zwsp;Char | Aliases
`BIG5` | Big Five | Traditional Chinese | No | No | 1–2 | `WIN950`, `Windows950`
`EUC_CN` | Extended UNIX Code-CN | Simplified Chinese | Yes | Yes | 1–3 |
`EUC_JP` | Extended UNIX Code-JP | Japanese | Yes | Yes | 1–3 |
`EUC_JIS_2004` | Extended UNIX Code-JP, JIS X 0213 | Japanese | Yes | No | 1–3 |
`EUC_KR` | Extended UNIX Code-KR | Korean | Yes | Yes | 1–3 |
`EUC_TW` | Extended UNIX Code-TW | Traditional Chinese, Taiwanese | Yes | Yes | 1–4 |
`GB18030` | National Standard | Chinese | No | No | 1–4 |
`GBK` | Extended National Standard | Simplified Chinese | No | No | 1–2 | `WIN936`, `Windows936`
`ISO_8859_5` | ISO 8859-5, ECMA 113 | Latin/Cyrillic | Yes | Yes | 1 |
`ISO_8859_6` | ISO 8859-6, ECMA 114 | Latin/Arabic | Yes | Yes | 1 |
`ISO_8859_7` | ISO 8859-7, ECMA 118 | Latin/Greek | Yes | Yes | 1 |
`ISO_8859_8` | ISO 8859-8, ECMA 121 | Latin/Hebrew | Yes | Yes | 1 |
`JOHAB` | JOHAB | Korean (Hangul) | No | No | 1–3 |
`KOI8R` | KOI8-R | Cyrillic (Russian) | Yes | Yes | 1 | `KOI8`
`KOI8U` | KOI8-U | Cyrillic (Ukrainian) | Yes | Yes | 1 |
`LATIN1` | ISO 8859-1, ECMA 94 | Western European | Yes | Yes | 1 | `ISO88591`
`LATIN2` | ISO 8859-2, ECMA 94 | Central European | Yes | Yes | 1 | `ISO88592`
`LATIN3` | ISO 8859-3, ECMA 94 | South European | Yes | Yes | 1 | `ISO88593`
`LATIN4` | ISO 8859-4, ECMA 94 | North European | Yes | Yes | 1 | `ISO88594`
`LATIN5` | ISO 8859-9, ECMA 128 | Turkish | Yes | Yes | 1 | `ISO88599`
`LATIN6` | ISO 8859-10, ECMA 144 | Nordic | Yes | Yes | 1 | `ISO885910`
`LATIN7` | ISO 8859-13 | Baltic | Yes | Yes | 1 | `ISO885913`
`LATIN8` | ISO 8859-14 | Celtic | Yes | Yes | 1 | `ISO885914`
`LATIN9` | ISO 8859-15 | LATIN1 with Euro and accents | Yes | Yes | 1 | `ISO885915`
`LATIN10` | ISO 8859-16, ASRO SR 14111 | Romanian | Yes | No | 1 | `ISO885916`
`MULE_INTERNAL` | Mule internal code | Multilingual Emacs | Yes | No | 1–4 |
`SJIS` | Shift JIS | Japanese | No | No | 1–2 | `Mskanji`, `ShiftJIS`, `WIN932`, `Windows932`
`SHIFT_JIS_2004` | Shift JIS, JIS X 0213 | Japanese | No | No | 1–2 |
`SQL_ASCII` | unspecified (see text) | any | Yes | No | 1 |
`UHC` | Unified Hangul Code | Korean | No | No | 1–2 | `WIN949`, `Windows949`
`UTF8` | Unicode, 8-bit | all | Yes | Yes | 1–4 | `Unicode`
`WIN866` | Windows CP866 | Cyrillic | Yes | Yes | 1 | `ALT`
`WIN874` | Windows CP874 | Thai | Yes | No | 1 |
`WIN1250` | Windows CP1250 | Central European | Yes | Yes | 1 |
`WIN1251` | Windows CP1251 | Cyrillic | Yes | Yes | 1 | `WIN`
`WIN1252` | Windows CP1252 | Western European | Yes | Yes | 1 |
`WIN1253` | Windows CP1253 | Greek | Yes | Yes | 1 |
`WIN1254` | Windows CP1254 | Turkish | Yes | Yes | 1 |
`WIN1255` | Windows CP1255 | Hebrew | Yes | Yes | 1 |
`WIN1256` | Windows CP1256 | Arabic | Yes | Yes | 1 |
`WIN1257` | Windows CP1257 | Baltic | Yes | Yes | 1 |
`WIN1258` | Windows CP1258 | Vietnamese | Yes | Yes | 1 | `ABC`, `TCVN`, `TCVN5712`, `VSCII`

Not all client APIs support all the listed character sets. For example, the PostgreSQL JDBC driver does not support `MULE_INTERNAL`, `LATIN6`, `LATIN8`, and `LATIN10`.

The `SQL_ASCII` setting behaves considerably differently from the other settings. When the server character set is `SQL_ASCII`, the server interprets byte values 0–127 according to the ASCII standard, while byte values 128–255 are taken as uninterpreted characters. No encoding conversion will be done when the setting is `SQL_ASCII`. Thus, this setting is not so much a declaration that a specific encoding is in use, as a declaration of ignorance about the encoding. In most cases, if you are working with any non-ASCII data, it is unwise to use the `SQL_ASCII` setting because PostgreSQL will be unable to help you by converting or validating non-ASCII characters.

### Setting the Character Set

`initdb` defines the default character set (encoding) for a PostgreSQL cluster. For example,

    initdb -E EUC_JP

sets the default character set to `EUC_JP` (Extended Unix Code for Japanese). You can use `--encoding` instead of `-E` if you prefer longer option strings. If no `-E` or `--encoding` option is given, `initdb` attempts to determine the appropriate encoding to use based on the specified or default locale.

You can specify a non-default encoding at database creation time, provided that the encoding is compatible with the selected locale:

    createdb -E EUC_KR -T template0 --lc-collate=ko_KR.euckr --lc-ctype=ko_KR.euckr korean

This will create a database named `korean` that uses the character set `EUC_KR`, and locale `ko_KR`. Another way to accomplish this is to use this SQL command:

    CREATE DATABASE korean WITH ENCODING 'EUC_KR' LC_COLLATE='ko_KR.euckr' LC_CTYPE='ko_KR.euckr' TEMPLATE=template0;

Notice that the above commands specify copying the `template0` database. When copying any other database, the encoding and locale settings cannot be changed from those of the source database, because that might result in corrupt data. For more information see manage-ag-templatedbs.

The encoding for a database is stored in the system catalog `pg_database`. You can see it by using the `psql` `-l` option or the `\l` command.

    $ psql -l
                                             List of databases
       Name    |  Owner   | Encoding  |  Collation  |    Ctype    |          Access Privileges
    -----------+----------+-----------+-------------+-------------+-------------------------------------
     clocaledb | hlinnaka | SQL_ASCII | C           | C           |
     englishdb | hlinnaka | UTF8      | en_GB.UTF8  | en_GB.UTF8  |
     japanese  | hlinnaka | UTF8      | ja_JP.UTF8  | ja_JP.UTF8  |
     korean    | hlinnaka | EUC_KR    | ko_KR.euckr | ko_KR.euckr |
     postgres  | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  |
     template0 | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
     template1 | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
    (7 rows)

On most modern operating systems, PostgreSQL can determine which character set is implied by the `LC_CTYPE` setting, and it will enforce that only the matching database encoding is used. On older systems it is your responsibility to ensure that you use the encoding expected by the locale you have selected. A mistake in this area is likely to lead to strange behavior of locale-dependent operations such as sorting.

PostgreSQL will allow superusers to create databases with `SQL_ASCII` encoding even when `LC_CTYPE` is not `C` or `POSIX`. As noted above, `SQL_ASCII` does not enforce that the data stored in the database has any particular encoding, and so this choice poses risks of locale-dependent misbehavior. Using this combination of settings is deprecated and may someday be forbidden altogether.

### Automatic Character Set Conversion Between Server and Client

PostgreSQL supports automatic character set conversion between server and client for many combinations of character sets (multibyte-conversions-supported shows which ones).

To enable automatic character set conversion, you have to tell PostgreSQL the character set (encoding) you would like to use in the client. There are several ways to accomplish this:

- Using the `\encoding` command in psql. `\encoding` allows you to change client encoding on the fly. For example, to change the encoding to `SJIS`, type:

      \encoding SJIS

- libpq (libpq-control) has functions to control the client encoding.

- Using `SET client_encoding TO`. Setting the client encoding can be done with this SQL command:

      SET CLIENT_ENCODING TO 'value';

  Also you can use the standard SQL syntax `SET NAMES` for this purpose:

      SET NAMES 'value';

  To query the current client encoding:

      SHOW client_encoding;

  To return to the default encoding:

      RESET client_encoding;

- Using `PGCLIENTENCODING`. If the environment variable `PGCLIENTENCODING` is defined in the client's environment, that client encoding is automatically selected when a connection to the server is made. (This can subsequently be overridden using any of the other methods mentioned above.)

- Using the configuration variable guc-client-encoding. If the `client_encoding` variable is set, that client encoding is automatically selected when a connection to the server is made. (This can subsequently be overridden using any of the other methods mentioned above.)

If the conversion of a particular character is not possible — suppose you chose `EUC_JP` for the server and `LATIN1` for the client, and some Japanese characters are returned that do not have a representation in `LATIN1` — an error is reported.

If the client character set is defined as `SQL_ASCII`, encoding conversion is disabled, regardless of the server's character set. (However, if the server's character set is not `SQL_ASCII`, the server will still check that incoming data is valid for that encoding; so the net effect is as though the client character set were the same as the server's.) Just as for the server, use of `SQL_ASCII` is unwise unless you are working with all-ASCII data.

### Available Character Set Conversions

PostgreSQL allows conversion between any two character sets for which a conversion function is listed in the pg_conversion system catalog. PostgreSQL comes with some predefined conversions, as summarized in multibyte-translation-table and shown in more detail in builtin-conversions-table. You can create a new conversion using the SQL command sql-createconversion. (To be used for automatic client/server conversions, a conversion must be marked as default for its character set pair.)

### Built-in Client/Server Character Set Conversions

Server Character Set | Available Client Character Sets
`BIG5` | not supported as a server encoding
`EUC_CN` | EUC_CN, `MULE_INTERNAL`, `UTF8`
`EUC_JP` | EUC_JP, `MULE_INTERNAL`, `SJIS`, `UTF8`
`EUC_JIS_2004` | EUC_JIS_2004, `SHIFT_JIS_2004`, `UTF8`
`EUC_KR` | EUC_KR, `MULE_INTERNAL`, `UTF8`
`EUC_TW` | EUC_TW, `BIG5`, `MULE_INTERNAL`, `UTF8`
`GB18030` | not supported as a server encoding
`GBK` | not supported as a server encoding
`ISO_8859_5` | ISO_8859_5, `KOI8R`, `MULE_INTERNAL`, `UTF8`, `WIN866`, `WIN1251`
`ISO_8859_6` | ISO_8859_6, `UTF8`
`ISO_8859_7` | ISO_8859_7, `UTF8`
`ISO_8859_8` | ISO_8859_8, `UTF8`
`JOHAB` | not supported as a server encoding
`KOI8R` | KOI8R, `ISO_8859_5`, `MULE_INTERNAL`, `UTF8`, `WIN866`, `WIN1251`
`KOI8U` | KOI8U, `UTF8`
`LATIN1` | LATIN1, `MULE_INTERNAL`, `UTF8`
`LATIN2` | LATIN2, `MULE_INTERNAL`, `UTF8`, `WIN1250`
`LATIN3` | LATIN3, `MULE_INTERNAL`, `UTF8`
`LATIN4` | LATIN4, `MULE_INTERNAL`, `UTF8`
`LATIN5` | LATIN5, `UTF8`
`LATIN6` | LATIN6, `UTF8`
`LATIN7` | LATIN7, `UTF8`
`LATIN8` | LATIN8, `UTF8`
`LATIN9` | LATIN9, `UTF8`
`LATIN10` | LATIN10, `UTF8`
`MULE_INTERNAL` | MULE_INTERNAL, `BIG5`, `EUC_CN`, `EUC_JP`, `EUC_KR`, `EUC_TW`, `ISO_8859_5`, `KOI8R`, `LATIN1` to `LATIN4`, `SJIS`, `WIN866`, `WIN1250`, `WIN1251`
`SJIS` | not supported as a server encoding
`SHIFT_JIS_2004` | not supported as a server encoding
`SQL_ASCII` | any (no conversion will be performed)
`UHC` | not supported as a server encoding
`UTF8` | all supported encodings
`WIN866` | WIN866, `ISO_8859_5`, `KOI8R`, `MULE_INTERNAL`, `UTF8`, `WIN1251`
`WIN874` | WIN874, `UTF8`
`WIN1250` | WIN1250, `LATIN2`, `MULE_INTERNAL`, `UTF8`
`WIN1251` | WIN1251, `ISO_8859_5`, `KOI8R`, `MULE_INTERNAL`, `UTF8`, `WIN866`
`WIN1252` | WIN1252, `UTF8`
`WIN1253` | WIN1253, `UTF8`
`WIN1254` | WIN1254, `UTF8`
`WIN1255` | WIN1255, `UTF8`
`WIN1256` | WIN1256, `UTF8`
`WIN1257` | WIN1257, `UTF8`
`WIN1258` | WIN1258, `UTF8`

### All Built-in Character Set Conversions

Conversion Name The conversion names follow a standard naming scheme: The official name of the source encoding with all non-alphanumeric characters replaced by underscores, followed by `_to_`, followed by the similarly processed destination encoding name. Therefore, these names sometimes deviate from the customary encoding names shown in charset-table. | Source Encoding | Destination Encoding
`big5_to_euc_tw` | `BIG5` | `EUC_TW`
`big5_to_mic` | `BIG5` | `MULE_INTERNAL`
`big5_to_utf8` | `BIG5` | `UTF8`
`euc_cn_to_mic` | `EUC_CN` | `MULE_INTERNAL`
`euc_cn_to_utf8` | `EUC_CN` | `UTF8`
`euc_jp_to_mic` | `EUC_JP` | `MULE_INTERNAL`
`euc_jp_to_sjis` | `EUC_JP` | `SJIS`
`euc_jp_to_utf8` | `EUC_JP` | `UTF8`
`euc_kr_to_mic` | `EUC_KR` | `MULE_INTERNAL`
`euc_kr_to_utf8` | `EUC_KR` | `UTF8`
`euc_tw_to_big5` | `EUC_TW` | `BIG5`
`euc_tw_to_mic` | `EUC_TW` | `MULE_INTERNAL`
`euc_tw_to_utf8` | `EUC_TW` | `UTF8`
`gb18030_to_utf8` | `GB18030` | `UTF8`
`gbk_to_utf8` | `GBK` | `UTF8`
`iso_8859_10_to_utf8` | `LATIN6` | `UTF8`
`iso_8859_13_to_utf8` | `LATIN7` | `UTF8`
`iso_8859_14_to_utf8` | `LATIN8` | `UTF8`
`iso_8859_15_to_utf8` | `LATIN9` | `UTF8`
`iso_8859_16_to_utf8` | `LATIN10` | `UTF8`
`iso_8859_1_to_mic` | `LATIN1` | `MULE_INTERNAL`
`iso_8859_1_to_utf8` | `LATIN1` | `UTF8`
`iso_8859_2_to_mic` | `LATIN2` | `MULE_INTERNAL`
`iso_8859_2_to_utf8` | `LATIN2` | `UTF8`
`iso_8859_2_to_windows_1250` | `LATIN2` | `WIN1250`
`iso_8859_3_to_mic` | `LATIN3` | `MULE_INTERNAL`
`iso_8859_3_to_utf8` | `LATIN3` | `UTF8`
`iso_8859_4_to_mic` | `LATIN4` | `MULE_INTERNAL`
`iso_8859_4_to_utf8` | `LATIN4` | `UTF8`
`iso_8859_5_to_koi8_r` | `ISO_8859_5` | `KOI8R`
`iso_8859_5_to_mic` | `ISO_8859_5` | `MULE_INTERNAL`
`iso_8859_5_to_utf8` | `ISO_8859_5` | `UTF8`
`iso_8859_5_to_windows_1251` | `ISO_8859_5` | `WIN1251`
`iso_8859_5_to_windows_866` | `ISO_8859_5` | `WIN866`
`iso_8859_6_to_utf8` | `ISO_8859_6` | `UTF8`
`iso_8859_7_to_utf8` | `ISO_8859_7` | `UTF8`
`iso_8859_8_to_utf8` | `ISO_8859_8` | `UTF8`
`iso_8859_9_to_utf8` | `LATIN5` | `UTF8`
`johab_to_utf8` | `JOHAB` | `UTF8`
`koi8_r_to_iso_8859_5` | `KOI8R` | `ISO_8859_5`
`koi8_r_to_mic` | `KOI8R` | `MULE_INTERNAL`
`koi8_r_to_utf8` | `KOI8R` | `UTF8`
`koi8_r_to_windows_1251` | `KOI8R` | `WIN1251`
`koi8_r_to_windows_866` | `KOI8R` | `WIN866`
`koi8_u_to_utf8` | `KOI8U` | `UTF8`
`mic_to_big5` | `MULE_INTERNAL` | `BIG5`
`mic_to_euc_cn` | `MULE_INTERNAL` | `EUC_CN`
`mic_to_euc_jp` | `MULE_INTERNAL` | `EUC_JP`
`mic_to_euc_kr` | `MULE_INTERNAL` | `EUC_KR`
`mic_to_euc_tw` | `MULE_INTERNAL` | `EUC_TW`
`mic_to_iso_8859_1` | `MULE_INTERNAL` | `LATIN1`
`mic_to_iso_8859_2` | `MULE_INTERNAL` | `LATIN2`
`mic_to_iso_8859_3` | `MULE_INTERNAL` | `LATIN3`
`mic_to_iso_8859_4` | `MULE_INTERNAL` | `LATIN4`
`mic_to_iso_8859_5` | `MULE_INTERNAL` | `ISO_8859_5`
`mic_to_koi8_r` | `MULE_INTERNAL` | `KOI8R`
`mic_to_sjis` | `MULE_INTERNAL` | `SJIS`
`mic_to_windows_1250` | `MULE_INTERNAL` | `WIN1250`
`mic_to_windows_1251` | `MULE_INTERNAL` | `WIN1251`
`mic_to_windows_866` | `MULE_INTERNAL` | `WIN866`
`sjis_to_euc_jp` | `SJIS` | `EUC_JP`
`sjis_to_mic` | `SJIS` | `MULE_INTERNAL`
`sjis_to_utf8` | `SJIS` | `UTF8`
`windows_1258_to_utf8` | `WIN1258` | `UTF8`
`uhc_to_utf8` | `UHC` | `UTF8`
`utf8_to_big5` | `UTF8` | `BIG5`
`utf8_to_euc_cn` | `UTF8` | `EUC_CN`
`utf8_to_euc_jp` | `UTF8` | `EUC_JP`
`utf8_to_euc_kr` | `UTF8` | `EUC_KR`
`utf8_to_euc_tw` | `UTF8` | `EUC_TW`
`utf8_to_gb18030` | `UTF8` | `GB18030`
`utf8_to_gbk` | `UTF8` | `GBK`
`utf8_to_iso_8859_1` | `UTF8` | `LATIN1`
`utf8_to_iso_8859_10` | `UTF8` | `LATIN6`
`utf8_to_iso_8859_13` | `UTF8` | `LATIN7`
`utf8_to_iso_8859_14` | `UTF8` | `LATIN8`
`utf8_to_iso_8859_15` | `UTF8` | `LATIN9`
`utf8_to_iso_8859_16` | `UTF8` | `LATIN10`
`utf8_to_iso_8859_2` | `UTF8` | `LATIN2`
`utf8_to_iso_8859_3` | `UTF8` | `LATIN3`
`utf8_to_iso_8859_4` | `UTF8` | `LATIN4`
`utf8_to_iso_8859_5` | `UTF8` | `ISO_8859_5`
`utf8_to_iso_8859_6` | `UTF8` | `ISO_8859_6`
`utf8_to_iso_8859_7` | `UTF8` | `ISO_8859_7`
`utf8_to_iso_8859_8` | `UTF8` | `ISO_8859_8`
`utf8_to_iso_8859_9` | `UTF8` | `LATIN5`
`utf8_to_johab` | `UTF8` | `JOHAB`
`utf8_to_koi8_r` | `UTF8` | `KOI8R`
`utf8_to_koi8_u` | `UTF8` | `KOI8U`
`utf8_to_sjis` | `UTF8` | `SJIS`
`utf8_to_windows_1258` | `UTF8` | `WIN1258`
`utf8_to_uhc` | `UTF8` | `UHC`
`utf8_to_windows_1250` | `UTF8` | `WIN1250`
`utf8_to_windows_1251` | `UTF8` | `WIN1251`
`utf8_to_windows_1252` | `UTF8` | `WIN1252`
`utf8_to_windows_1253` | `UTF8` | `WIN1253`
`utf8_to_windows_1254` | `UTF8` | `WIN1254`
`utf8_to_windows_1255` | `UTF8` | `WIN1255`
`utf8_to_windows_1256` | `UTF8` | `WIN1256`
`utf8_to_windows_1257` | `UTF8` | `WIN1257`
`utf8_to_windows_866` | `UTF8` | `WIN866`
`utf8_to_windows_874` | `UTF8` | `WIN874`
`windows_1250_to_iso_8859_2` | `WIN1250` | `LATIN2`
`windows_1250_to_mic` | `WIN1250` | `MULE_INTERNAL`
`windows_1250_to_utf8` | `WIN1250` | `UTF8`
`windows_1251_to_iso_8859_5` | `WIN1251` | `ISO_8859_5`
`windows_1251_to_koi8_r` | `WIN1251` | `KOI8R`
`windows_1251_to_mic` | `WIN1251` | `MULE_INTERNAL`
`windows_1251_to_utf8` | `WIN1251` | `UTF8`
`windows_1251_to_windows_866` | `WIN1251` | `WIN866`
`windows_1252_to_utf8` | `WIN1252` | `UTF8`
`windows_1256_to_utf8` | `WIN1256` | `UTF8`
`windows_866_to_iso_8859_5` | `WIN866` | `ISO_8859_5`
`windows_866_to_koi8_r` | `WIN866` | `KOI8R`
`windows_866_to_mic` | `WIN866` | `MULE_INTERNAL`
`windows_866_to_utf8` | `WIN866` | `UTF8`
`windows_866_to_windows_1251` | `WIN866` | `WIN`
`windows_874_to_utf8` | `WIN874` | `UTF8`
`euc_jis_2004_to_utf8` | `EUC_JIS_2004` | `UTF8`
`utf8_to_euc_jis_2004` | `UTF8` | `EUC_JIS_2004`
`shift_jis_2004_to_utf8` | `SHIFT_JIS_2004` | `UTF8`
`utf8_to_shift_jis_2004` | `UTF8` | `SHIFT_JIS_2004`
`euc_jis_2004_to_shift_jis_2004` | `EUC_JIS_2004` | `SHIFT_JIS_2004`
`shift_jis_2004_to_euc_jis_2004` | `SHIFT_JIS_2004` | `EUC_JIS_2004`

### Further Reading

These are good sources to start learning about various kinds of encoding systems.

CJKV Information Processing: Chinese, Japanese, Korean & Vietnamese Computing

- Contains detailed explanations of `EUC_JP`, `EUC_CN`, `EUC_KR`, `EUC_TW`.

- The web site of the Unicode Consortium.

RFC 3629

- UTF-8 (8-bit UCS/Unicode Transformation Format) is defined here.