v14-0005-Introduce-collation-provider-builtin-for-C-and-C.patch

From 00117afe789db96daf39f6a8e2fa4b68c4469b35 Mon Sep 17 00:00:00 2001
From: Jeff Davis <jeff@j-davis.com>
Date: Mon, 1 May 2023 15:38:29 -0700
Subject: [PATCH v14 5/5] Introduce collation provider "builtin" for "C" and
 "C.UTF-8".

The builtin "C" locale is equal (in semantics and implementation) to
the libc "C" locale, except that LC_COLLATE and LC_CTYPE can be
configured independently.

The builtin "C.UTF-8" locale is especially useful. It provides a fast
memcmp-based collation (like "C") that supports abbrevated keys, while
also providing richer ctype semantics (upper/lower and regexes). The
semantics are derived from Unicode by building in lookup tables in the
same way as for text normalization. By using built-in semantics, the
behavior is stabilized within a Postgres major version, and also
matches the behavior of other built-in Unicode functionality, such as
normalization.

Discussion: https://postgr.es/m/ab925f69-5f9d-f85e-b87c-bd2a44798659@joeconway.com
Discussion: https://postgr.es/m/dd9261f4-7a98-4565-93ec-336c1c110d90@manitou-mail.org
---
 doc/src/sgml/charset.sgml                    |  88 ++++++++++---
 doc/src/sgml/ref/create_collation.sgml       |  11 +-
 doc/src/sgml/ref/create_database.sgml        |   8 +-
 doc/src/sgml/ref/createdb.sgml               |   2 +-
 doc/src/sgml/ref/initdb.sgml                 |   7 +-
 src/backend/catalog/pg_collation.c           |   5 +-
 src/backend/commands/collationcmds.c         |  91 +++++++++++---
 src/backend/commands/dbcommands.c            | 105 ++++++++++++----
 src/backend/regex/regc_pg_locale.c           |  41 ++++++-
 src/backend/utils/adt/formatting.c           | 122 +++++++++++++++++++
 src/backend/utils/adt/pg_locale.c            | 107 +++++++++++++---
 src/backend/utils/init/postinit.c            |  27 +++-
 src/bin/initdb/initdb.c                      |  16 ++-
 src/bin/initdb/t/001_initdb.pl               |  47 +++++++
 src/bin/pg_dump/pg_dump.c                    |  15 ++-
 src/bin/pg_upgrade/t/002_pg_upgrade.pl       |  25 +++-
 src/bin/psql/describe.c                      |   4 +-
 src/bin/scripts/createdb.c                   |   2 +-
 src/bin/scripts/t/020_createdb.pl            |  56 +++++++++
 src/include/catalog/pg_collation.dat         |   4 +-
 src/include/catalog/pg_collation.h           |   3 +
 src/include/utils/pg_locale.h                |   7 +-
 src/test/icu/t/010_database.pl               |  18 +--
 src/test/regress/expected/collate.out        |  24 +++-
 src/test/regress/expected/collate.utf8.out   |  93 ++++++++++++++
 src/test/regress/expected/collate.utf8_1.out |   8 ++
 src/test/regress/parallel_schedule           |   4 +-
 src/test/regress/sql/collate.sql             |  10 ++
 src/test/regress/sql/collate.utf8.sql        |  47 +++++++
 29 files changed, 885 insertions(+), 112 deletions(-)
 create mode 100644 src/test/regress/expected/collate.utf8.out
 create mode 100644 src/test/regress/expected/collate.utf8_1.out
 create mode 100644 src/test/regress/sql/collate.utf8.sql

diff --git a/doc/src/sgml/charset.sgml b/doc/src/sgml/charset.sgml
index 74783d148f..51315ceac7 100644
--- a/doc/src/sgml/charset.sgml
+++ b/doc/src/sgml/charset.sgml
@@ -342,22 +342,14 @@ initdb --locale=sv_SE
    <title>Locale Providers</title>
 
    <para>
-    <productname>PostgreSQL</productname> supports multiple <firstterm>locale
-    providers</firstterm>.  This specifies which library supplies the locale
-    data.  One standard provider name is <literal>libc</literal>, 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 <literal>icu</literal>, which uses the external
-    ICU<indexterm><primary>ICU</primary></indexterm> library.  ICU locales can
-    only be used if support for ICU was configured when PostgreSQL was built.
+    A locale provider specifies which library defines the locale behavior for
+    collations and character classifications.
    </para>
 
    <para>
     The commands and tools that select the locale settings, as described
-    above, each have an option to select the locale provider.  The examples
-    shown earlier all use the <literal>libc</literal> provider, which is the
-    default.  Here is an example to initialize a database cluster using the
-    ICU provider:
+    above, each have an option to select the locale provider. Here is an
+    example to initialize a database cluster using the ICU provider:
 <programlisting>
 initdb --locale-provider=icu --icu-locale=en
 </programlisting>
@@ -370,12 +362,74 @@ initdb --locale-provider=icu --icu-locale=en
    </para>
 
    <para>
-    Which locale provider to use depends on individual requirements.  For most
-    basic uses, either provider will give adequate results.  For the libc
-    provider, it depends on what the operating system offers; some operating
-    systems are better than others.  For advanced uses, ICU offers more locale
-    variants and customization options.
+    Regardless of the locale provider, the operating system is still used to
+    provide some locale-aware behavior, such as messages (see <xref
+    linkend="guc-lc-messages"/>).
    </para>
+
+   <para>
+    The available locale providers are listed below.
+   </para>
+
+   <sect3 id="locale-provider-builtin">
+    <title>Builtin</title>
+    <para>
+     The <literal>builtin</literal> provider uses built-in operations. Only
+     the <literal>C</literal> and <literal>C.UTF-8</literal> locales are
+     supported for this provider.
+    </para>
+    <para>
+     The collation and character classification behavior is equivalent to
+     using the <literal>libc</literal> provider with locale
+     <literal>C</literal>, except that <literal>LC_COLLATE</literal> and
+     <literal>LC_CTYPE</literal> can be set independently.
+    </para>
+    <note>
+     <para>
+      When using the <literal>builtin</literal> locale provider, behavior may
+      depend on the database encoding.
+     </para>
+    </note>
+   </sect3>
+   <sect3 id="locale-provider-icu">
+    <title>ICU</title>
+    <para>
+     The <literal>icu</literal> provider uses the external
+     ICU<indexterm><primary>ICU</primary></indexterm>
+     library. <productname>PostgreSQL</productname> must have been configured
+     with support.
+    </para>
+    <para>
+     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. <literal>LC_COLLATE</literal> and
+     <literal>LC_CTYPE</literal> can be set independently of the ICU locale.
+    </para>
+    <note>
+     <para>
+      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.
+     </para>
+    </note>
+   </sect3>
+   <sect3 id="locale-provider-libc">
+    <title>libc</title>
+    <para>
+     The <literal>libc</literal> provider uses the operating system's C
+     library. The collation and character classification behavior is
+     controlled by the settings <literal>LC_COLLATE</literal> and
+     <literal>LC_CTYPE</literal>, so they cannot be set independently.
+    </para>
+    <note>
+     <para>
+      The same locale name may have different behavior on different platforms
+      when using the libc provider.
+     </para>
+    </note>
+   </sect3>
+
   </sect2>
 
   <sect2 id="icu-locales">
diff --git a/doc/src/sgml/ref/create_collation.sgml b/doc/src/sgml/ref/create_collation.sgml
index 5cf9777764..85f18cbbe5 100644
--- a/doc/src/sgml/ref/create_collation.sgml
+++ b/doc/src/sgml/ref/create_collation.sgml
@@ -96,6 +96,11 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
        <replaceable>locale</replaceable>, you cannot specify either of those
        parameters.
       </para>
+      <para>
+       If <replaceable>provider</replaceable> is <literal>builtin</literal>,
+       then <replaceable>locale</replaceable> must be specified and set to
+       either <literal>C</literal> or <literal>C.UTF-8</literal>.
+      </para>
      </listitem>
     </varlistentry>
 
@@ -129,9 +134,9 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Specifies the provider to use for locale services associated with this
-       collation.  Possible values are
-       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
-       (if the server was built with ICU support) or <literal>libc</literal>.
+       collation.  Possible values are <literal>builtin</literal>,
+       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm> (if
+       the server was built with ICU support) or <literal>libc</literal>.
        <literal>libc</literal> is the default.  See <xref
        linkend="locale-providers"/> for details.
       </para>
diff --git a/doc/src/sgml/ref/create_database.sgml b/doc/src/sgml/ref/create_database.sgml
index ce7317f81b..6dc3348d1b 100644
--- a/doc/src/sgml/ref/create_database.sgml
+++ b/doc/src/sgml/ref/create_database.sgml
@@ -162,6 +162,12 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         linkend="create-database-lc-ctype"/>, or <xref
         linkend="create-database-icu-locale"/> individually.
        </para>
+       <para>
+        If <xref linkend="create-database-locale-provider"/> is
+        <literal>builtin</literal>, then <replaceable>locale</replaceable>
+        must be specified and set to either <literal>C</literal> or
+        <literal>C.UTF-8</literal>.
+       </para>
        <tip>
         <para>
          The other locale settings <xref linkend="guc-lc-messages"/>, <xref
@@ -243,7 +249,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
       <listitem>
        <para>
         Specifies the provider to use for the default collation in this
-        database.  Possible values are
+        database.  Possible values are <literal>builtin</literal>,
         <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
         (if the server was built with ICU support) or <literal>libc</literal>.
         By default, the provider is the same as that of the <xref
diff --git a/doc/src/sgml/ref/createdb.sgml b/doc/src/sgml/ref/createdb.sgml
index e4647d5ce7..d3e815f659 100644
--- a/doc/src/sgml/ref/createdb.sgml
+++ b/doc/src/sgml/ref/createdb.sgml
@@ -171,7 +171,7 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-      <term><option>--locale-provider={<literal>libc</literal>|<literal>icu</literal>}</option></term>
+      <term><option>--locale-provider={<literal>builtin</literal>|<literal>libc</literal>|<literal>icu</literal>}</option></term>
       <listitem>
        <para>
         Specifies the locale provider for the database's default collation.
diff --git a/doc/src/sgml/ref/initdb.sgml b/doc/src/sgml/ref/initdb.sgml
index d43c91575c..2192d017b3 100644
--- a/doc/src/sgml/ref/initdb.sgml
+++ b/doc/src/sgml/ref/initdb.sgml
@@ -286,6 +286,11 @@ PostgreSQL documentation
         environment that <command>initdb</command> runs in. Locale
         support is described in <xref linkend="locale"/>.
        </para>
+       <para>
+        If <option>--locale-provider</option> is <literal>builtin</literal>,
+        <option>--locale</option> must be specified and set to
+        <literal>C</literal> or <literal>C.UTF-8</literal>.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -315,7 +320,7 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry id="app-initdb-option-locale-provider">
-      <term><option>--locale-provider={<literal>libc</literal>|<literal>icu</literal>}</option></term>
+      <term><option>--locale-provider={<literal>builtin</literal>|<literal>libc</literal>|<literal>icu</literal>}</option></term>
       <listitem>
        <para>
         This option sets the locale provider for databases created in the new
diff --git a/src/backend/catalog/pg_collation.c b/src/backend/catalog/pg_collation.c
index b615d2fc7a..2144e11fe8 100644
--- a/src/backend/catalog/pg_collation.c
+++ b/src/backend/catalog/pg_collation.c
@@ -68,7 +68,10 @@ CollationCreate(const char *collname, Oid collnamespace,
 	Assert(collname);
 	Assert(collnamespace);
 	Assert(collowner);
-	Assert((collcollate && collctype) || colllocale);
+	Assert((collprovider == COLLPROVIDER_LIBC &&
+			 collcollate &&  collctype && !colllocale) ||
+		   (collprovider != COLLPROVIDER_LIBC &&
+			!collcollate && !collctype &&  colllocale));
 
 	/*
 	 * Make sure there is no existing collation of same name & encoding.
diff --git a/src/backend/commands/collationcmds.c b/src/backend/commands/collationcmds.c
index 19e61b3e5b..3514cd9d3d 100644
--- a/src/backend/commands/collationcmds.c
+++ b/src/backend/commands/collationcmds.c
@@ -215,7 +215,9 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 
 		if (collproviderstr)
 		{
-			if (pg_strcasecmp(collproviderstr, "icu") == 0)
+			if (pg_strcasecmp(collproviderstr, "builtin") == 0)
+				collprovider = COLLPROVIDER_BUILTIN;
+			else if (pg_strcasecmp(collproviderstr, "icu") == 0)
 				collprovider = COLLPROVIDER_ICU;
 			else if (pg_strcasecmp(collproviderstr, "libc") == 0)
 				collprovider = COLLPROVIDER_LIBC;
@@ -245,7 +247,17 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 		if (lcctypeEl)
 			collctype = defGetString(lcctypeEl);
 
-		if (collprovider == COLLPROVIDER_LIBC)
+		if (collprovider == COLLPROVIDER_BUILTIN)
+		{
+			if (!colllocale)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("parameter \"locale\" must be specified")));
+
+			colllocale = builtin_validate_locale(GetDatabaseEncoding(),
+												 colllocale);
+		}
+		else if (collprovider == COLLPROVIDER_LIBC)
 		{
 			if (!collcollate)
 				ereport(ERROR,
@@ -305,7 +317,17 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 					 errmsg("ICU rules cannot be specified unless locale provider is ICU")));
 
-		if (collprovider == COLLPROVIDER_ICU)
+		if (collprovider == COLLPROVIDER_BUILTIN)
+		{
+			/*
+			 * Behavior may be different in different encodings, so set
+			 * collencoding to the current database encoding. No validation is
+			 * required, because the "builtin" provider is compatible with any
+			 * encoding.
+			 */
+			collencoding = GetDatabaseEncoding();
+		}
+		else if (collprovider == COLLPROVIDER_ICU)
 		{
 #ifdef USE_ICU
 			/*
@@ -334,7 +356,18 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
 	}
 
 	if (!collversion)
-		collversion = get_collation_actual_version(collprovider, collprovider == COLLPROVIDER_ICU ? colllocale : collcollate);
+	{
+		char *locale;
+
+		if (collprovider == COLLPROVIDER_ICU)
+			locale = colllocale;
+		else if (collprovider == COLLPROVIDER_LIBC)
+			locale = collcollate;
+		else
+			locale = NULL; /* COLLPROVIDER_BUILTIN */
+
+		collversion = get_collation_actual_version(collprovider, locale);
+	}
 
 	newoid = CollationCreate(collName,
 							 collNamespace,
@@ -409,6 +442,7 @@ AlterCollation(AlterCollationStmt *stmt)
 	Form_pg_collation collForm;
 	Datum		datum;
 	bool		isnull;
+	char	   *locale;
 	char	   *oldversion;
 	char	   *newversion;
 	ObjectAddress address;
@@ -435,8 +469,20 @@ AlterCollation(AlterCollationStmt *stmt)
 	datum = SysCacheGetAttr(COLLOID, tup, Anum_pg_collation_collversion, &isnull);
 	oldversion = isnull ? NULL : TextDatumGetCString(datum);
 
-	datum = SysCacheGetAttrNotNull(COLLOID, tup, collForm->collprovider == COLLPROVIDER_ICU ? Anum_pg_collation_colllocale : Anum_pg_collation_collcollate);
-	newversion = get_collation_actual_version(collForm->collprovider, TextDatumGetCString(datum));
+	if (collForm->collprovider == COLLPROVIDER_ICU)
+	{
+		datum = SysCacheGetAttrNotNull(COLLOID, tup, Anum_pg_collation_colllocale);
+		locale = TextDatumGetCString(datum);
+	}
+	else if (collForm->collprovider == COLLPROVIDER_LIBC)
+	{
+		datum = SysCacheGetAttrNotNull(COLLOID, tup, Anum_pg_collation_collcollate);
+		locale = TextDatumGetCString(datum);
+	}
+	else
+		locale = NULL; /* COLLPROVIDER_BUILTIN */
+
+	newversion = get_collation_actual_version(collForm->collprovider, locale);
 
 	/* cannot change from NULL to non-NULL or vice versa */
 	if ((!oldversion && newversion) || (oldversion && !newversion))
@@ -500,11 +546,18 @@ pg_collation_actual_version(PG_FUNCTION_ARGS)
 
 		provider = ((Form_pg_database) GETSTRUCT(dbtup))->datlocprovider;
 
-		datum = SysCacheGetAttrNotNull(DATABASEOID, dbtup,
-									   provider == COLLPROVIDER_ICU ?
-									   Anum_pg_database_datlocale : Anum_pg_database_datcollate);
-
-		locale = TextDatumGetCString(datum);
+		if (provider == COLLPROVIDER_ICU)
+		{
+			datum = SysCacheGetAttrNotNull(DATABASEOID, dbtup, Anum_pg_database_datlocale);
+			locale = TextDatumGetCString(datum);
+		}
+		else if (provider == COLLPROVIDER_LIBC)
+		{
+			datum = SysCacheGetAttrNotNull(DATABASEOID, dbtup, Anum_pg_database_datcollate);
+			locale = TextDatumGetCString(datum);
+		}
+		else
+			locale = NULL; /* COLLPROVIDER_BUILTIN */
 
 		ReleaseSysCache(dbtup);
 	}
@@ -521,11 +574,19 @@ pg_collation_actual_version(PG_FUNCTION_ARGS)
 
 		provider = ((Form_pg_collation) GETSTRUCT(colltp))->collprovider;
 		Assert(provider != COLLPROVIDER_DEFAULT);
-		datum = SysCacheGetAttrNotNull(COLLOID, colltp,
-									   provider == COLLPROVIDER_ICU ?
-									   Anum_pg_collation_colllocale : Anum_pg_collation_collcollate);
 
-		locale = TextDatumGetCString(datum);
+		if (provider == COLLPROVIDER_ICU)
+		{
+			datum = SysCacheGetAttrNotNull(COLLOID, colltp, Anum_pg_collation_colllocale);
+			locale = TextDatumGetCString(datum);
+		}
+		else if (provider == COLLPROVIDER_LIBC)
+		{
+			datum = SysCacheGetAttrNotNull(COLLOID, colltp, Anum_pg_collation_collcollate);
+			locale = TextDatumGetCString(datum);
+		}
+		else
+			locale = NULL; /* COLLPROVIDER_BUILTIN */
 
 		ReleaseSysCache(colltp);
 	}
diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index c8732be067..eaec22e771 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -897,6 +897,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	{
 		dbcollate = defGetString(dlocale);
 		dbctype = defGetString(dlocale);
+		dblocale = defGetString(dlocale);
 	}
 	if (dcollate && dcollate->arg)
 		dbcollate = defGetString(dcollate);
@@ -910,7 +911,9 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	{
 		char	   *locproviderstr = defGetString(dlocprovider);
 
-		if (pg_strcasecmp(locproviderstr, "icu") == 0)
+		if (pg_strcasecmp(locproviderstr, "builtin") == 0)
+			dblocprovider = COLLPROVIDER_BUILTIN;
+		else if (pg_strcasecmp(locproviderstr, "icu") == 0)
 			dblocprovider = COLLPROVIDER_ICU;
 		else if (pg_strcasecmp(locproviderstr, "libc") == 0)
 			dblocprovider = COLLPROVIDER_LIBC;
@@ -1027,14 +1030,9 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 		dbctype = src_ctype;
 	if (dblocprovider == '\0')
 		dblocprovider = src_locprovider;
-	if (dblocale == NULL && dblocprovider == COLLPROVIDER_ICU)
-	{
-		if (dlocale && dlocale->arg)
-			dblocale = defGetString(dlocale);
-		else
-			dblocale = src_locale;
-	}
-	if (dbicurules == NULL && dblocprovider == COLLPROVIDER_ICU)
+	if (dblocale == NULL)
+		dblocale = src_locale;
+	if (dbicurules == NULL)
 		dbicurules = src_icurules;
 
 	/* Some encodings are client only */
@@ -1059,6 +1057,20 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 
 	check_encoding_locale_matches(encoding, dbcollate, dbctype);
 
+	if (dblocprovider == COLLPROVIDER_BUILTIN)
+	{
+		/*
+		 * This would happen if template0 uses the libc provider but the new
+		 * database uses builtin.
+		 */
+		if (!dblocale)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("LOCALE must be specified for the builtin provider")));
+
+		dblocale = builtin_validate_locale(encoding, dblocale);
+	}
+
 	if (dblocprovider == COLLPROVIDER_ICU)
 	{
 		if (!(is_encoding_supported_by_icu(encoding)))
@@ -1100,7 +1112,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	}
 	else
 	{
-		if (dblocale)
+		if (diculocale && diculocale->arg)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
 					 errmsg("ICU locale cannot be specified unless locale provider is ICU")));
@@ -1111,6 +1123,10 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 					 errmsg("ICU rules cannot be specified unless locale provider is ICU")));
 	}
 
+	/* for libc, locale comes from datcollate and datctype */
+	if (dblocprovider == COLLPROVIDER_LIBC)
+		dblocale = NULL;
+
 	/*
 	 * Check that the new encoding and locale settings match the source
 	 * database.  We insist on this because we simply copy the source data ---
@@ -1195,9 +1211,17 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 */
 	if (src_collversion && !dcollversion)
 	{
-		char	   *actual_versionstr;
+		char	*actual_versionstr;
+		char	*locale;
 
-		actual_versionstr = get_collation_actual_version(dblocprovider, dblocprovider == COLLPROVIDER_ICU ? dblocale : dbcollate);
+		if (dblocprovider == COLLPROVIDER_ICU)
+			locale = dblocale;
+		else if (dblocprovider == COLLPROVIDER_LIBC)
+			locale = dbcollate;
+		else
+			locale = NULL; /* COLLPROVIDER_BUILTIN */
+
+		actual_versionstr = get_collation_actual_version(dblocprovider, locale);
 		if (!actual_versionstr)
 			ereport(ERROR,
 					(errmsg("template database \"%s\" has a collation version, but no actual collation version could be determined",
@@ -1225,7 +1249,18 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 * collation version, which is normally only the case for template0.
 	 */
 	if (dbcollversion == NULL)
-		dbcollversion = get_collation_actual_version(dblocprovider, dblocprovider == COLLPROVIDER_ICU ? dblocale : dbcollate);
+	{
+		char *locale;
+
+		if (dblocprovider == COLLPROVIDER_ICU)
+			locale = dblocale;
+		else if (dblocprovider == COLLPROVIDER_LIBC)
+			locale = dbcollate;
+		else
+			locale = NULL; /* COLLPROVIDER_BUILTIN */
+
+		dbcollversion = get_collation_actual_version(dblocprovider, locale);
+	}
 
 	/* Resolve default tablespace for new database */
 	if (dtablespacename && dtablespacename->arg)
@@ -1364,8 +1399,8 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 * block on the unique index, and fail after we commit).
 	 */
 
-	Assert((dblocprovider == COLLPROVIDER_ICU && dblocale) ||
-		   (dblocprovider != COLLPROVIDER_ICU && !dblocale));
+	Assert((dblocprovider != COLLPROVIDER_LIBC && dblocale) ||
+		   (dblocprovider == COLLPROVIDER_LIBC && !dblocale));
 
 	/* Form tuple */
 	new_record[Anum_pg_database_oid - 1] = ObjectIdGetDatum(dboid);
@@ -2446,6 +2481,7 @@ AlterDatabaseRefreshColl(AlterDatabaseRefreshCollStmt *stmt)
 	ObjectAddress address;
 	Datum		datum;
 	bool		isnull;
+	char	   *locale;
 	char	   *oldversion;
 	char	   *newversion;
 
@@ -2472,10 +2508,24 @@ AlterDatabaseRefreshColl(AlterDatabaseRefreshCollStmt *stmt)
 	datum = heap_getattr(tuple, Anum_pg_database_datcollversion, RelationGetDescr(rel), &isnull);
 	oldversion = isnull ? NULL : TextDatumGetCString(datum);
 
-	datum = heap_getattr(tuple, datForm->datlocprovider == COLLPROVIDER_ICU ? Anum_pg_database_datlocale : Anum_pg_database_datcollate, RelationGetDescr(rel), &isnull);
-	if (isnull)
-		elog(ERROR, "unexpected null in pg_database");
-	newversion = get_collation_actual_version(datForm->datlocprovider, TextDatumGetCString(datum));
+	if (datForm->datlocprovider == COLLPROVIDER_ICU)
+	{
+		datum = heap_getattr(tuple, Anum_pg_database_datlocale, RelationGetDescr(rel), &isnull);
+		if (isnull)
+			elog(ERROR, "unexpected null in pg_database");
+		locale = TextDatumGetCString(datum);
+	}
+	else if (datForm->datlocprovider == COLLPROVIDER_LIBC)
+	{
+		datum = heap_getattr(tuple, Anum_pg_database_datcollate, RelationGetDescr(rel), &isnull);
+		if (isnull)
+			elog(ERROR, "unexpected null in pg_database");
+		locale = TextDatumGetCString(datum);
+	}
+	else
+		locale = NULL; /* COLLPROVIDER_BUILTIN */
+
+	newversion = get_collation_actual_version(datForm->datlocprovider, locale);
 
 	/* cannot change from NULL to non-NULL or vice versa */
 	if ((!oldversion && newversion) || (oldversion && !newversion))
@@ -2660,6 +2710,7 @@ pg_database_collation_actual_version(PG_FUNCTION_ARGS)
 	HeapTuple	tp;
 	char		datlocprovider;
 	Datum		datum;
+	char	   *locale;
 	char	   *version;
 
 	tp = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(dbid));
@@ -2670,8 +2721,20 @@ pg_database_collation_actual_version(PG_FUNCTION_ARGS)
 
 	datlocprovider = ((Form_pg_database) GETSTRUCT(tp))->datlocprovider;
 
-	datum = SysCacheGetAttrNotNull(DATABASEOID, tp, datlocprovider == COLLPROVIDER_ICU ? Anum_pg_database_datlocale : Anum_pg_database_datcollate);
-	version = get_collation_actual_version(datlocprovider, TextDatumGetCString(datum));
+	if (datlocprovider == COLLPROVIDER_ICU)
+	{
+		datum = SysCacheGetAttrNotNull(DATABASEOID, tp, Anum_pg_database_datlocale);
+		locale = TextDatumGetCString(datum);
+	}
+	else if (datlocprovider == COLLPROVIDER_LIBC)
+	{
+		datum = SysCacheGetAttrNotNull(DATABASEOID, tp, Anum_pg_database_datcollate);
+		locale = TextDatumGetCString(datum);
+	}
+	else
+		locale = NULL; /* COLLPROVIDER_BUILTIN */
+
+	version = get_collation_actual_version(datlocprovider, locale);
 
 	ReleaseSysCache(tp);
 
diff --git a/src/backend/regex/regc_pg_locale.c b/src/backend/regex/regc_pg_locale.c
index 42d15b6303..0278f45adc 100644
--- a/src/backend/regex/regc_pg_locale.c
+++ b/src/backend/regex/regc_pg_locale.c
@@ -16,6 +16,8 @@
  */
 
 #include "catalog/pg_collation.h"
+#include "common/unicode_case.h"
+#include "common/unicode_category.h"
 #include "utils/pg_locale.h"
 
 /*
@@ -64,6 +66,7 @@
 typedef enum
 {
 	PG_REGEX_LOCALE_C,			/* C locale (encoding independent) */
+	PG_REGEX_BUILTIN,			/* built-in Unicode semantics */
 	PG_REGEX_LOCALE_WIDE,		/* Use <wctype.h> functions */
 	PG_REGEX_LOCALE_1BYTE,		/* Use <ctype.h> functions */
 	PG_REGEX_LOCALE_WIDE_L,		/* Use locale_t <wctype.h> functions */
@@ -75,6 +78,8 @@ static PG_Locale_Strategy pg_regex_strategy;
 static pg_locale_t pg_regex_locale;
 static Oid	pg_regex_collation;
 
+static bool regex_builtin_cclass_posix = false;
+
 /*
  * Hard-wired character properties for C locale
  */
@@ -266,7 +271,15 @@ pg_set_regex_collation(Oid collation)
 		if (GetDatabaseEncoding() == PG_UTF8)
 		{
 			if (pg_regex_locale)
-				pg_regex_strategy = PG_REGEX_LOCALE_WIDE_L;
+			{
+				if (pg_regex_locale->provider == COLLPROVIDER_BUILTIN)
+				{
+					pg_regex_strategy = PG_REGEX_BUILTIN;
+					regex_builtin_cclass_posix = pg_regex_locale->info.builtin.cclass_posix;
+				}
+				else
+					pg_regex_strategy = PG_REGEX_LOCALE_WIDE_L;
+			}
 			else
 				pg_regex_strategy = PG_REGEX_LOCALE_WIDE;
 		}
@@ -290,6 +303,8 @@ pg_wc_isdigit(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISDIGIT));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isdigit(c, regex_builtin_cclass_posix);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswdigit((wint_t) c);
@@ -322,6 +337,8 @@ pg_wc_isalpha(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISALPHA));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isalpha(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalpha((wint_t) c);
@@ -354,6 +371,8 @@ pg_wc_isalnum(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISALNUM));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isalnum(c, regex_builtin_cclass_posix);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalnum((wint_t) c);
@@ -395,6 +414,8 @@ pg_wc_isupper(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISUPPER));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isupper(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswupper((wint_t) c);
@@ -427,6 +448,8 @@ pg_wc_islower(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISLOWER));
+		case PG_REGEX_BUILTIN:
+			return pg_u_islower(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswlower((wint_t) c);
@@ -459,6 +482,8 @@ pg_wc_isgraph(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISGRAPH));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isgraph(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswgraph((wint_t) c);
@@ -491,6 +516,8 @@ pg_wc_isprint(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISPRINT));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isprint(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswprint((wint_t) c);
@@ -523,6 +550,8 @@ pg_wc_ispunct(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISPUNCT));
+		case PG_REGEX_BUILTIN:
+			return pg_u_ispunct(c, regex_builtin_cclass_posix);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswpunct((wint_t) c);
@@ -555,6 +584,8 @@ pg_wc_isspace(pg_wchar c)
 		case PG_REGEX_LOCALE_C:
 			return (c <= (pg_wchar) 127 &&
 					(pg_char_properties[c] & PG_ISSPACE));
+		case PG_REGEX_BUILTIN:
+			return pg_u_isspace(c);
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswspace((wint_t) c);
@@ -588,6 +619,8 @@ pg_wc_toupper(pg_wchar c)
 			if (c <= (pg_wchar) 127)
 				return pg_ascii_toupper((unsigned char) c);
 			return c;
+		case PG_REGEX_BUILTIN:
+			return unicode_uppercase_simple(c);
 		case PG_REGEX_LOCALE_WIDE:
 			/* force C behavior for ASCII characters, per comments above */
 			if (c <= (pg_wchar) 127)
@@ -628,6 +661,8 @@ pg_wc_tolower(pg_wchar c)
 			if (c <= (pg_wchar) 127)
 				return pg_ascii_tolower((unsigned char) c);
 			return c;
+		case PG_REGEX_BUILTIN:
+			return unicode_lowercase_simple(c);
 		case PG_REGEX_LOCALE_WIDE:
 			/* force C behavior for ASCII characters, per comments above */
 			if (c <= (pg_wchar) 127)
@@ -792,6 +827,9 @@ pg_ctype_get_cache(pg_wc_probefunc probefunc, int cclasscode)
 			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
 #endif
 			break;
+		case PG_REGEX_BUILTIN:
+			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
+			break;
 		case PG_REGEX_LOCALE_WIDE:
 		case PG_REGEX_LOCALE_WIDE_L:
 			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
@@ -809,6 +847,7 @@ pg_ctype_get_cache(pg_wc_probefunc probefunc, int cclasscode)
 			max_chr = (pg_wchar) MAX_SIMPLE_CHR;
 			break;
 		default:
+			Assert(false);
 			max_chr = 0;		/* can't get here, but keep compiler quiet */
 			break;
 	}
diff --git a/src/backend/utils/adt/formatting.c b/src/backend/utils/adt/formatting.c
index d176723d95..cf50b4d951 100644
--- a/src/backend/utils/adt/formatting.c
+++ b/src/backend/utils/adt/formatting.c
@@ -77,6 +77,8 @@
 
 #include "catalog/pg_collation.h"
 #include "catalog/pg_type.h"
+#include "common/unicode_case.h"
+#include "common/unicode_category.h"
 #include "mb/pg_wchar.h"
 #include "nodes/miscnodes.h"
 #include "parser/scansup.h"
@@ -1670,6 +1672,43 @@ str_tolower(const char *buff, size_t nbytes, Oid collid)
 		}
 		else
 #endif
+		if (mylocale && mylocale->provider == COLLPROVIDER_BUILTIN)
+		{
+			const unsigned char *orig = (unsigned char *) buff;
+			unsigned char *workspace;
+			const unsigned char *sp;
+			unsigned char *rp;
+
+			Assert(GetDatabaseEncoding() == PG_UTF8);
+
+			/* Overflow paranoia */
+			if ((nbytes + 1) > (INT_MAX / sizeof(pg_wchar)))
+				ereport(ERROR,
+						(errcode(ERRCODE_OUT_OF_MEMORY),
+						 errmsg("out of memory")));
+
+			/* Output workspace cannot have more codes than input bytes */
+			workspace = (unsigned char *)palloc((nbytes + 1) * sizeof(pg_wchar));
+
+			sp = orig;
+			rp = workspace;
+			while (sp - orig < nbytes)
+			{
+				pg_wchar u = utf8_to_unicode(sp);
+				unicode_to_utf8(unicode_lowercase_simple(u), rp);
+				sp += pg_utf_mblen(sp);
+				rp += pg_utf_mblen(rp);
+			}
+
+			*rp = '\0';
+			rp++;
+
+			/* shrink buffer and store result */
+			result = palloc(rp - workspace);
+			memcpy(result, workspace, rp - workspace);
+			pfree(workspace);
+		}
+		else
 		{
 			if (pg_database_encoding_max_length() > 1)
 			{
@@ -1788,6 +1827,43 @@ str_toupper(const char *buff, size_t nbytes, Oid collid)
 		}
 		else
 #endif
+		if (mylocale && mylocale->provider == COLLPROVIDER_BUILTIN)
+		{
+			const unsigned char *orig = (unsigned char *) buff;
+			unsigned char *workspace;
+			const unsigned char *sp;
+			unsigned char *rp;
+
+			Assert(GetDatabaseEncoding() == PG_UTF8);
+
+			/* Overflow paranoia */
+			if ((nbytes + 1) > (INT_MAX / sizeof(pg_wchar)))
+				ereport(ERROR,
+						(errcode(ERRCODE_OUT_OF_MEMORY),
+						 errmsg("out of memory")));
+
+			/* Output workspace cannot have more codes than input bytes */
+			workspace = (unsigned char *)palloc((nbytes + 1) * sizeof(pg_wchar));
+
+			sp = orig;
+			rp = workspace;
+			while (sp - orig < nbytes)
+			{
+				pg_wchar u = utf8_to_unicode(sp);
+				unicode_to_utf8(unicode_uppercase_simple(u), rp);
+				sp += pg_utf_mblen(sp);
+				rp += pg_utf_mblen(rp);
+			}
+
+			*rp = '\0';
+			rp++;
+
+			/* shrink buffer and store result */
+			result = palloc(rp - workspace);
+			memcpy(result, workspace, rp - workspace);
+			pfree(workspace);
+		}
+		else
 		{
 			if (pg_database_encoding_max_length() > 1)
 			{
@@ -1907,6 +1983,52 @@ str_initcap(const char *buff, size_t nbytes, Oid collid)
 		}
 		else
 #endif
+		if (mylocale && mylocale->provider == COLLPROVIDER_BUILTIN)
+		{
+			const unsigned char *orig = (unsigned char *) buff;
+			unsigned char *workspace;
+			const unsigned char *sp;
+			unsigned char *rp;
+
+			Assert(GetDatabaseEncoding() == PG_UTF8);
+
+			/* Overflow paranoia */
+			if ((nbytes + 1) > (INT_MAX / sizeof(pg_wchar)))
+				ereport(ERROR,
+						(errcode(ERRCODE_OUT_OF_MEMORY),
+						 errmsg("out of memory")));
+
+			/* Output workspace cannot have more codes than input bytes */
+			workspace = (unsigned char *)palloc((nbytes + 1) * sizeof(pg_wchar));
+
+			sp = orig;
+			rp = workspace;
+			while (sp - orig < nbytes)
+			{
+				pg_wchar u1 = utf8_to_unicode(sp);
+				pg_wchar u2;
+
+				if (wasalnum)
+					u2 = unicode_lowercase_simple(u1);
+				else
+					u2 = unicode_titlecase_simple(u1);
+
+				unicode_to_utf8(u2, rp);
+				wasalnum = pg_u_isalnum(u2, mylocale->info.builtin.cclass_posix);
+
+				sp += pg_utf_mblen(sp);
+				rp += pg_utf_mblen(rp);
+			}
+
+			*rp = '\0';
+			rp++;
+
+			/* shrink buffer and store result */
+			result = palloc(rp - workspace);
+			memcpy(result, workspace, rp - workspace);
+			pfree(workspace);
+		}
+		else
 		{
 			if (pg_database_encoding_max_length() > 1)
 			{
diff --git a/src/backend/utils/adt/pg_locale.c b/src/backend/utils/adt/pg_locale.c
index 9d8634ec25..4ae0b8eb67 100644
--- a/src/backend/utils/adt/pg_locale.c
+++ b/src/backend/utils/adt/pg_locale.c
@@ -1269,7 +1269,19 @@ lookup_collation_cache(Oid collation, bool set_flags)
 			elog(ERROR, "cache lookup failed for collation %u", collation);
 		collform = (Form_pg_collation) GETSTRUCT(tp);
 
-		if (collform->collprovider == COLLPROVIDER_LIBC)
+		if (collform->collprovider == COLLPROVIDER_BUILTIN)
+		{
+			Datum		datum;
+			const char *colllocale;
+
+			datum = SysCacheGetAttrNotNull(COLLOID, tp, Anum_pg_collation_colllocale);
+			colllocale = TextDatumGetCString(datum);
+
+			cache_entry->collate_is_c = true;
+			cache_entry->ctype_is_c = ((strcmp(colllocale, "C") == 0) ||
+									   (strcmp(colllocale, "POSIX") == 0));
+		}
+		else if (collform->collprovider == COLLPROVIDER_LIBC)
 		{
 			Datum		datum;
 			const char *collcollate;
@@ -1320,16 +1332,29 @@ lc_collate_is_c(Oid collation)
 	if (collation == DEFAULT_COLLATION_OID)
 	{
 		static int	result = -1;
-		char	   *localeptr;
-
-		if (default_locale.provider == COLLPROVIDER_ICU)
-			return false;
+		const char *localeptr;
 
 		if (result >= 0)
 			return (bool) result;
-		localeptr = setlocale(LC_COLLATE, NULL);
-		if (!localeptr)
-			elog(ERROR, "invalid LC_COLLATE setting");
+
+		if (default_locale.provider == COLLPROVIDER_BUILTIN)
+		{
+			localeptr = default_locale.info.builtin.locale;
+		}
+		else if (default_locale.provider == COLLPROVIDER_ICU)
+		{
+			result = false;
+			return (bool) result;
+		}
+		else if (default_locale.provider == COLLPROVIDER_LIBC)
+		{
+			localeptr = setlocale(LC_CTYPE, NULL);
+			if (!localeptr)
+				elog(ERROR, "invalid LC_CTYPE setting");
+		}
+		else
+			elog(ERROR, "unexpected collation provider '%c'",
+				 default_locale.provider);
 
 		if (strcmp(localeptr, "C") == 0)
 			result = true;
@@ -1373,16 +1398,29 @@ lc_ctype_is_c(Oid collation)
 	if (collation == DEFAULT_COLLATION_OID)
 	{
 		static int	result = -1;
-		char	   *localeptr;
-
-		if (default_locale.provider == COLLPROVIDER_ICU)
-			return false;
+		const char *localeptr;
 
 		if (result >= 0)
 			return (bool) result;
-		localeptr = setlocale(LC_CTYPE, NULL);
-		if (!localeptr)
-			elog(ERROR, "invalid LC_CTYPE setting");
+
+		if (default_locale.provider == COLLPROVIDER_BUILTIN)
+		{
+			localeptr = default_locale.info.builtin.locale;
+		}
+		else if (default_locale.provider == COLLPROVIDER_ICU)
+		{
+			result = false;
+			return (bool) result;
+		}
+		else if (default_locale.provider == COLLPROVIDER_LIBC)
+		{
+			localeptr = setlocale(LC_CTYPE, NULL);
+			if (!localeptr)
+				elog(ERROR, "invalid LC_CTYPE setting");
+		}
+		else
+			elog(ERROR, "unexpected collation provider '%c'",
+				 default_locale.provider);
 
 		if (strcmp(localeptr, "C") == 0)
 			result = true;
@@ -1390,6 +1428,7 @@ lc_ctype_is_c(Oid collation)
 			result = true;
 		else
 			result = false;
+
 		return (bool) result;
 	}
 
@@ -1520,10 +1559,10 @@ pg_newlocale_from_collation(Oid collid)
 
 	if (collid == DEFAULT_COLLATION_OID)
 	{
-		if (default_locale.provider == COLLPROVIDER_ICU)
-			return &default_locale;
-		else
+		if (default_locale.provider == COLLPROVIDER_LIBC)
 			return (pg_locale_t) 0;
+		else
+			return &default_locale;
 	}
 
 	cache_entry = lookup_collation_cache(collid, false);
@@ -1548,7 +1587,18 @@ pg_newlocale_from_collation(Oid collid)
 		result.provider = collform->collprovider;
 		result.deterministic = collform->collisdeterministic;
 
-		if (collform->collprovider == COLLPROVIDER_LIBC)
+		if (collform->collprovider == COLLPROVIDER_BUILTIN)
+		{
+			const char *locstr;
+
+			datum = SysCacheGetAttrNotNull(COLLOID, tp, Anum_pg_collation_colllocale);
+			locstr = TextDatumGetCString(datum);
+
+			result.info.builtin.locale = MemoryContextStrdup(TopMemoryContext,
+															 locstr);
+			result.info.builtin.cclass_posix = true;
+		}
+		else if (collform->collprovider == COLLPROVIDER_LIBC)
 		{
 			const char *collcollate;
 			const char *collctype pg_attribute_unused();
@@ -1627,6 +1677,7 @@ pg_newlocale_from_collation(Oid collid)
 
 			collversionstr = TextDatumGetCString(datum);
 
+			Assert(collform->collprovider != COLLPROVIDER_BUILTIN);
 			datum = SysCacheGetAttrNotNull(COLLOID, tp, collform->collprovider == COLLPROVIDER_ICU ? Anum_pg_collation_colllocale : Anum_pg_collation_collcollate);
 
 			actual_versionstr = get_collation_actual_version(collform->collprovider,
@@ -1678,6 +1729,9 @@ get_collation_actual_version(char collprovider, const char *collcollate)
 {
 	char	   *collversion = NULL;
 
+	if (collprovider == COLLPROVIDER_BUILTIN)
+		return NULL; /* TODO */
+
 #ifdef USE_ICU
 	if (collprovider == COLLPROVIDER_ICU)
 	{
@@ -2444,6 +2498,21 @@ pg_strnxfrm_prefix(char *dest, size_t destsize, const char *src,
 	return result;
 }
 
+char *
+builtin_validate_locale(int encoding, const char *locale)
+{
+	if (strcmp(locale, "C") == 0 || strcmp(locale, "POSIX") == 0)
+		return "C";
+	if (encoding == PG_UTF8 &&
+		(strcmp(locale, "C.UTF-8") == 0 || strcmp(locale, "C.UTF8") == 0))
+		return "C.UTF-8";
+
+	ereport(ERROR,
+			(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+			 errmsg("invalid locale name \"%s\" for builtin provider", locale)));
+}
+
+
 #ifdef USE_ICU
 
 /*
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 702bad6d8a..f6dc3afbfe 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -318,7 +318,7 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 	bool		isnull;
 	char	   *collate;
 	char	   *ctype;
-	char	   *iculocale;
+	char	   *datlocale;
 
 	/* Fetch our pg_database row normally, via syscache */
 	tup = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
@@ -423,12 +423,21 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 		strcmp(ctype, "POSIX") == 0)
 		database_ctype_is_c = true;
 
-	if (dbform->datlocprovider == COLLPROVIDER_ICU)
+	if (dbform->datlocprovider == COLLPROVIDER_BUILTIN)
+	{
+		datum = SysCacheGetAttrNotNull(DATABASEOID, tup, Anum_pg_database_datlocale);
+		datlocale = TextDatumGetCString(datum);
+
+		default_locale.info.builtin.locale = MemoryContextStrdup(
+			TopMemoryContext, datlocale);
+		default_locale.info.builtin.cclass_posix = true;
+	}
+	else if (dbform->datlocprovider == COLLPROVIDER_ICU)
 	{
 		char	   *icurules;
 
 		datum = SysCacheGetAttrNotNull(DATABASEOID, tup, Anum_pg_database_datlocale);
-		iculocale = TextDatumGetCString(datum);
+		datlocale = TextDatumGetCString(datum);
 
 		datum = SysCacheGetAttr(DATABASEOID, tup, Anum_pg_database_daticurules, &isnull);
 		if (!isnull)
@@ -436,10 +445,10 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 		else
 			icurules = NULL;
 
-		make_icu_collator(iculocale, icurules, &default_locale);
+		make_icu_collator(datlocale, icurules, &default_locale);
 	}
 	else
-		iculocale = NULL;
+		datlocale = NULL;
 
 	default_locale.provider = dbform->datlocprovider;
 
@@ -461,10 +470,16 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
 	{
 		char	   *actual_versionstr;
 		char	   *collversionstr;
+		char	   *locale;
 
 		collversionstr = TextDatumGetCString(datum);
 
-		actual_versionstr = get_collation_actual_version(dbform->datlocprovider, dbform->datlocprovider == COLLPROVIDER_ICU ? iculocale : collate);
+		if (dbform->datlocprovider == COLLPROVIDER_LIBC)
+			locale = collate;
+		else
+			locale = datlocale;
+
+		actual_versionstr = get_collation_actual_version(dbform->datlocprovider, locale);
 		if (!actual_versionstr)
 			/* should not happen */
 			elog(WARNING,
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index aaf55ce0cd..b075d0e2eb 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -2450,7 +2450,7 @@ usage(const char *progname)
 			 "                            set default locale in the respective category for\n"
 			 "                            new databases (default taken from environment)\n"));
 	printf(_("      --no-locale           equivalent to --locale=C\n"));
-	printf(_("      --locale-provider={libc|icu}\n"
+	printf(_("      --locale-provider={builtin|libc|icu}\n"
 			 "                            set default locale provider for new databases\n"));
 	printf(_("      --pwfile=FILE         read password for the new superuser from file\n"));
 	printf(_("  -T, --text-search-config=CFG\n"
@@ -2601,7 +2601,15 @@ setup_locale_encoding(void)
 {
 	setlocales();
 
-	if (locale_provider == COLLPROVIDER_LIBC &&
+	if (locale_provider == COLLPROVIDER_BUILTIN &&
+		strcmp(lc_ctype, "C") == 0 &&
+		strcmp(lc_collate, "C") == 0 &&
+		strcmp(lc_time, "C") == 0 &&
+		strcmp(lc_numeric, "C") == 0 &&
+		strcmp(lc_monetary, "C") == 0 &&
+		strcmp(lc_messages, "C") == 0)
+		printf(_("The database cluster will be initialized with no locale.\n"));
+	else if (locale_provider == COLLPROVIDER_LIBC &&
 		strcmp(lc_ctype, lc_collate) == 0 &&
 		strcmp(lc_ctype, lc_time) == 0 &&
 		strcmp(lc_ctype, lc_numeric) == 0 &&
@@ -3277,7 +3285,9 @@ main(int argc, char *argv[])
 										 "-c debug_discard_caches=1");
 				break;
 			case 15:
-				if (strcmp(optarg, "icu") == 0)
+				if (strcmp(optarg, "builtin") == 0)
+					locale_provider = COLLPROVIDER_BUILTIN;
+				else if (strcmp(optarg, "icu") == 0)
 					locale_provider = COLLPROVIDER_ICU;
 				else if (strcmp(optarg, "libc") == 0)
 					locale_provider = COLLPROVIDER_LIBC;
diff --git a/src/bin/initdb/t/001_initdb.pl b/src/bin/initdb/t/001_initdb.pl
index 7606db1987..5b8a253d02 100644
--- a/src/bin/initdb/t/001_initdb.pl
+++ b/src/bin/initdb/t/001_initdb.pl
@@ -184,6 +184,53 @@ else
 		'locale provider ICU fails since no ICU support');
 }
 
+command_fails(
+	[
+		'initdb', '--no-sync', '--locale-provider=builtin', "$tempdir/data6"
+	],
+	'locale provider builtin fails without --locale'
+);
+
+command_ok(
+	[
+		'initdb', '--no-sync', '--locale-provider=builtin', '--locale=C',
+		"$tempdir/data7"
+	],
+	'locale provider builtin with --locale'
+);
+
+command_ok(
+	[
+		'initdb', '--no-sync', '--locale-provider=builtin', '--lc-collate=C',
+		'--locale=C.UTF-8', "$tempdir/data8"
+	],
+	'locale provider builtin with --lc-collate and --locale=C.UTF-8'
+);
+
+command_ok(
+	[
+		'initdb', '--no-sync', '--locale-provider=builtin', '--lc-ctype=C',
+		'--locale=C', "$tempdir/data9"
+	],
+	'locale provider builtin with --lc-ctype'
+);
+
+command_fails(
+	[
+		'initdb', '--no-sync', '--locale-provider=builtin', '--icu-locale=en',
+		"$tempdir/dataX"
+	],
+	'fails for locale provider builtin with ICU locale'
+);
+
+command_fails(
+	[
+		'initdb', '--no-sync', '--locale-provider=builtin', '--icu-rules=""',
+		"$tempdir/dataX"
+	],
+	'fails for locale provider builtin with ICU rules'
+);
+
 command_fails(
 	[ 'initdb', '--no-sync', '--locale-provider=xyz', "$tempdir/dataX" ],
 	'fails for invalid locale provider');
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index 3d829c99d7..b953db8bea 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -3091,7 +3091,9 @@ dumpDatabase(Archive *fout)
 	}
 
 	appendPQExpBufferStr(creaQry, " LOCALE_PROVIDER = ");
-	if (datlocprovider[0] == 'c')
+	if (datlocprovider[0] == 'b')
+		appendPQExpBufferStr(creaQry, "builtin");
+	else if (datlocprovider[0] == 'c')
 		appendPQExpBufferStr(creaQry, "libc");
 	else if (datlocprovider[0] == 'i')
 		appendPQExpBufferStr(creaQry, "icu");
@@ -13672,7 +13674,9 @@ dumpCollation(Archive *fout, const CollInfo *collinfo)
 					  fmtQualifiedDumpable(collinfo));
 
 	appendPQExpBufferStr(q, "provider = ");
-	if (collprovider[0] == 'c')
+	if (collprovider[0] == 'b')
+		appendPQExpBufferStr(q, "builtin");
+	else if (collprovider[0] == 'c')
 		appendPQExpBufferStr(q, "libc");
 	else if (collprovider[0] == 'i')
 		appendPQExpBufferStr(q, "icu");
@@ -13693,6 +13697,13 @@ dumpCollation(Archive *fout, const CollInfo *collinfo)
 
 		/* no locale -- the default collation cannot be reloaded anyway */
 	}
+	else if (collprovider[0] == 'b')
+	{
+		if (collcollate || collctype || colllocale || collicurules)
+			pg_log_warning("invalid collation \"%s\"", qcollname);
+
+		appendPQExpBufferStr(q, ", locale = 'C'");
+	}
 	else if (collprovider[0] == 'i')
 	{
 		if (fout->remoteVersion >= 150000)
diff --git a/src/bin/pg_upgrade/t/002_pg_upgrade.pl b/src/bin/pg_upgrade/t/002_pg_upgrade.pl
index 6f359d72ce..c620a4929f 100644
--- a/src/bin/pg_upgrade/t/002_pg_upgrade.pl
+++ b/src/bin/pg_upgrade/t/002_pg_upgrade.pl
@@ -129,14 +129,33 @@ if (int($oldnode->pg_version) >= 15 && $ENV{with_icu} eq 'yes')
 	$original_iculocale = "fr-CA";
 }
 
+# use builtin provider instead of libc, if supported
+if ($oldnode->pg_version >= 16 && $ENV{with_icu} ne 'yes')
+{
+	$original_provider = "b";
+}
+
 my @initdb_params = @custom_opts;
 
 push @initdb_params, ('--encoding', 'UTF-8');
 push @initdb_params, ('--locale', $original_locale);
-if ($original_provider eq "i")
+
+# add --locale-provider, if supported
+if ($oldnode->pg_version >= 15)
 {
-	push @initdb_params, ('--locale-provider', 'icu');
-	push @initdb_params, ('--icu-locale', 'fr-CA');
+	if ($original_provider eq "b")
+	{
+		push @initdb_params, ('--locale-provider', 'builtin');
+	}
+	elsif ($original_provider eq "i")
+	{
+		push @initdb_params, ('--locale-provider', 'icu');
+		push @initdb_params, ('--icu-locale', 'fr-CA');
+	}
+	elsif ($original_provider eq "c")
+	{
+		push @initdb_params, ('--locale-provider', 'libc');
+	}
 }
 
 $node_params{extra} = \@initdb_params;
diff --git a/src/bin/psql/describe.c b/src/bin/psql/describe.c
index 1122843715..3b327d159a 100644
--- a/src/bin/psql/describe.c
+++ b/src/bin/psql/describe.c
@@ -926,7 +926,7 @@ listAllDbs(const char *pattern, bool verbose)
 					  gettext_noop("Encoding"));
 	if (pset.sversion >= 150000)
 		appendPQExpBuffer(&buf,
-						  "  CASE d.datlocprovider WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
+						  "  CASE d.datlocprovider WHEN 'b' THEN 'builtin' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
 						  gettext_noop("Locale Provider"));
 	else
 		appendPQExpBuffer(&buf,
@@ -4966,7 +4966,7 @@ listCollations(const char *pattern, bool verbose, bool showSystem)
 
 	if (pset.sversion >= 100000)
 		appendPQExpBuffer(&buf,
-						  "  CASE c.collprovider WHEN 'd' THEN 'default' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
+						  "  CASE c.collprovider WHEN 'd' THEN 'default' WHEN 'b' THEN 'builtin' WHEN 'c' THEN 'libc' WHEN 'i' THEN 'icu' END AS \"%s\",\n",
 						  gettext_noop("Provider"));
 	else
 		appendPQExpBuffer(&buf,
diff --git a/src/bin/scripts/createdb.c b/src/bin/scripts/createdb.c
index 9ca86a3e53..8f8995964c 100644
--- a/src/bin/scripts/createdb.c
+++ b/src/bin/scripts/createdb.c
@@ -296,7 +296,7 @@ help(const char *progname)
 	printf(_("      --lc-ctype=LOCALE        LC_CTYPE setting for the database\n"));
 	printf(_("      --icu-locale=LOCALE      ICU locale setting for the database\n"));
 	printf(_("      --icu-rules=RULES        ICU rules setting for the database\n"));
-	printf(_("      --locale-provider={libc|icu}\n"
+	printf(_("      --locale-provider={builtin|libc|icu}\n"
 			 "                               locale provider for the database's default collation\n"));
 	printf(_("  -O, --owner=OWNER            database user to own the new database\n"));
 	printf(_("  -S, --strategy=STRATEGY      database creation strategy wal_log or file_copy\n"));
diff --git a/src/bin/scripts/t/020_createdb.pl b/src/bin/scripts/t/020_createdb.pl
index 40291924e5..447cb7d2a3 100644
--- a/src/bin/scripts/t/020_createdb.pl
+++ b/src/bin/scripts/t/020_createdb.pl
@@ -105,6 +105,62 @@ else
 		'create database with ICU fails since no ICU support');
 }
 
+$node->command_fails(
+	[
+		'createdb', '-T', 'template0', '--locale-provider=builtin',
+		'tbuiltin1'
+	],
+	'create database with provider "builtin" fails without --locale'
+);
+
+$node->command_ok(
+	[
+		'createdb', '-T', 'template0', '--locale-provider=builtin',
+		'--locale=C', 'tbuiltin2'
+	],
+	'create database with provider "builtin" and locale "C"'
+);
+
+$node->command_ok(
+	[
+		'createdb', '-T', 'template0', '--locale-provider=builtin',
+		'--locale=C', '--lc-collate=C', 'tbuiltin3'
+	],
+	'create database with provider "builtin" and LC_COLLATE=C'
+);
+
+$node->command_ok(
+	[
+		'createdb', '-T', 'template0', '--locale-provider=builtin',
+		'--locale=C', '--lc-ctype=C', 'tbuiltin4'
+	],
+	'create database with provider "builtin" and LC_CTYPE=C'
+);
+
+$node->command_fails(
+	[
+		'createdb', '-T', 'template0', '--locale-provider=builtin',
+		'--locale=C', '--icu-locale=en', 'tbuiltin5'
+	],
+	'create database with provider "builtin" and ICU_LOCALE="en"'
+);
+
+$node->command_fails(
+	[
+		'createdb', '-T', 'template0', '--locale-provider=builtin',
+		'--locale=C', '--icu-rules=""', 'tbuiltin6'
+	],
+	'create database with provider "builtin" and ICU_RULES=""'
+);
+
+$node->command_fails(
+	[
+		'createdb', '-T', 'template1', '--locale-provider=builtin',
+		'--locale=C', 'tbuiltin7'
+	],
+	'create database with provider "builtin" not matching template'
+);
+
 $node->command_fails([ 'createdb', 'foobar1' ],
 	'fails if database already exists');
 
diff --git a/src/include/catalog/pg_collation.dat b/src/include/catalog/pg_collation.dat
index 2c112cd6bc..977f90518e 100644
--- a/src/include/catalog/pg_collation.dat
+++ b/src/include/catalog/pg_collation.dat
@@ -24,8 +24,8 @@
   collname => 'POSIX', collprovider => 'c', collencoding => '-1',
   collcollate => 'POSIX', collctype => 'POSIX' },
 { oid => '962', descr => 'sorts by Unicode code point',
-  collname => 'ucs_basic', collprovider => 'c', collencoding => '6',
-  collcollate => 'C', collctype => 'C' },
+  collname => 'ucs_basic', collprovider => 'b', collencoding => '6',
+  colllocale => 'C' },
 { oid => '963',
   descr => 'sorts using the Unicode Collation Algorithm with default settings',
   collname => 'unicode', collprovider => 'i', collencoding => '-1',
diff --git a/src/include/catalog/pg_collation.h b/src/include/catalog/pg_collation.h
index d357c89ae6..09fc991038 100644
--- a/src/include/catalog/pg_collation.h
+++ b/src/include/catalog/pg_collation.h
@@ -65,6 +65,7 @@ DECLARE_UNIQUE_INDEX_PKEY(pg_collation_oid_index, 3085, CollationOidIndexId, pg_
 #ifdef EXPOSE_TO_CLIENT_CODE
 
 #define COLLPROVIDER_DEFAULT	'd'
+#define COLLPROVIDER_BUILTIN	'b'
 #define COLLPROVIDER_ICU		'i'
 #define COLLPROVIDER_LIBC		'c'
 
@@ -73,6 +74,8 @@ collprovider_name(char c)
 {
 	switch (c)
 	{
+		case COLLPROVIDER_BUILTIN:
+			return "builtin";
 		case COLLPROVIDER_ICU:
 			return "icu";
 		case COLLPROVIDER_LIBC:
diff --git a/src/include/utils/pg_locale.h b/src/include/utils/pg_locale.h
index 6447bea8e0..688157b9ac 100644
--- a/src/include/utils/pg_locale.h
+++ b/src/include/utils/pg_locale.h
@@ -76,6 +76,11 @@ struct pg_locale_struct
 	bool		deterministic;
 	union
 	{
+		struct
+		{
+			const char *locale;
+			bool cclass_posix;
+		}			builtin;
 		locale_t	lt;
 #ifdef USE_ICU
 		struct
@@ -112,7 +117,7 @@ extern size_t pg_strxfrm_prefix(char *dest, const char *src, size_t destsize,
 								pg_locale_t locale);
 extern size_t pg_strnxfrm_prefix(char *dest, size_t destsize, const char *src,
 								 size_t srclen, pg_locale_t locale);
-
+extern char *builtin_validate_locale(int encoding, const char *loc_str);
 extern void icu_validate_locale(const char *loc_str);
 extern char *icu_language_tag(const char *loc_str, int elevel);
 
diff --git a/src/test/icu/t/010_database.pl b/src/test/icu/t/010_database.pl
index 67fc3bbf19..af34cfc1dd 100644
--- a/src/test/icu/t/010_database.pl
+++ b/src/test/icu/t/010_database.pl
@@ -63,14 +63,14 @@ is( $node1->psql(
 	0,
 	"C locale works for ICU");
 
-# Test that LOCALE works for ICU locales if LC_COLLATE and LC_CTYPE
-# are specified
-is( $node1->psql(
-		'postgres',
-		q{CREATE DATABASE dbicu2 LOCALE_PROVIDER icu LOCALE '@colStrength=primary'
-      LC_COLLATE='C' LC_CTYPE='C' TEMPLATE template0 ENCODING UTF8}
-	),
-	0,
-	"LOCALE works for ICU locales if LC_COLLATE and LC_CTYPE are specified");
+my ($ret, $stdout, $stderr) = $node1->psql('postgres',
+	q{CREATE DATABASE dbicu LOCALE_PROVIDER builtin LOCALE 'C' TEMPLATE dbicu}
+);
+isnt($ret, 0,
+	"locale provider must match template: exit code not 0");
+like(
+	$stderr,
+	qr/ERROR:  new locale provider \(builtin\) does not match locale provider of the template database \(icu\)/,
+	"locale provider must match template: error message");
 
 done_testing();
diff --git a/src/test/regress/expected/collate.out b/src/test/regress/expected/collate.out
index 0649564485..ece4a8e99d 100644
--- a/src/test/regress/expected/collate.out
+++ b/src/test/regress/expected/collate.out
@@ -650,6 +650,26 @@ EXPLAIN (COSTS OFF)
 (3 rows)
 
 -- CREATE/DROP COLLATION
+CREATE COLLATION builtin_c ( PROVIDER = builtin, LOCALE = "C" );
+CREATE COLLATION builtin_posix ( PROVIDER = builtin, LOCALE = "POSIX" );
+SELECT b FROM collate_test1 ORDER BY b COLLATE builtin_c;
+  b  
+-----
+ ABD
+ Abc
+ abc
+ bbc
+(4 rows)
+
+CREATE COLLATION builtin2 ( PROVIDER = builtin ); -- fails
+ERROR:  parameter "locale" must be specified
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "en_US" ); -- fails
+ERROR:  invalid locale name "en_US" for builtin provider
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LC_CTYPE = "C", LC_COLLATE = "C" ); -- fails
+ERROR:  parameter "locale" must be specified
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "POSIX", LC_CTYPE = "POSIX" ); -- fails
+ERROR:  conflicting or redundant options
+DETAIL:  LOCALE cannot be specified together with LC_COLLATE or LC_CTYPE.
 CREATE COLLATION mycoll1 FROM "C";
 CREATE COLLATION mycoll2 ( LC_COLLATE = "POSIX", LC_CTYPE = "POSIX" );
 CREATE COLLATION mycoll3 FROM "default";  -- intentionally unsupported
@@ -754,7 +774,7 @@ DETAIL:  FROM cannot be specified together with any other options.
 -- must get rid of them.
 --
 DROP SCHEMA collate_tests CASCADE;
-NOTICE:  drop cascades to 19 other objects
+NOTICE:  drop cascades to 21 other objects
 DETAIL:  drop cascades to table collate_test1
 drop cascades to table collate_test_like
 drop cascades to table collate_test2
@@ -771,6 +791,8 @@ drop cascades to function dup(anyelement)
 drop cascades to table collate_test20
 drop cascades to table collate_test21
 drop cascades to table collate_test22
+drop cascades to collation builtin_c
+drop cascades to collation builtin_posix
 drop cascades to collation mycoll2
 drop cascades to table collate_test23
 drop cascades to view collate_on_int
diff --git a/src/test/regress/expected/collate.utf8.out b/src/test/regress/expected/collate.utf8.out
new file mode 100644
index 0000000000..0c332f7896
--- /dev/null
+++ b/src/test/regress/expected/collate.utf8.out
@@ -0,0 +1,93 @@
+/*
+ * This test is for collations and character operations when using the
+ * builtin provider with the C.UTF-8 locale.
+ */
+/* skip test if not UTF8 server encoding */
+SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+SET client_encoding TO UTF8;
+--
+-- Test builtin provider with C.UTF-8 locale.
+--
+CREATE COLLATION builtin_c_utf8 (PROVIDER = builtin, LOCALE = 'C.UTF-8');
+CREATE TABLE builtin_test (
+  t TEXT COLLATE builtin_c_utf8
+);
+INSERT INTO builtin_test VALUES
+  ('abc DEF'),
+  ('DŽxxx Džxxx džxxx'),
+  ('xxxDŽ xxxDž xxxdž');
+SELECT t, lower(t), initcap(t), upper(t) FROM builtin_test;
+       t        |     lower      |    initcap     |     upper      
+----------------+----------------+----------------+----------------
+ abc DEF        | abc def        | Abc Def        | ABC DEF
+ DŽxxx Džxxx džxxx | džxxx džxxx džxxx | Džxxx Džxxx Džxxx | DŽXXX DŽXXX DŽXXX
+ xxxDŽ xxxDž xxxdž | xxxdž xxxdž xxxdž | Xxxdž Xxxdž Xxxdž | XXXDŽ XXXDŽ XXXDŽ
+(3 rows)
+
+DROP TABLE builtin_test;
+-- character classes
+select 'xyz' ~ '[[:alnum:]]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select 'xyz' !~ '[[:upper:]]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select '@' !~ '[[:alnum:]]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select '@' ~ '[[:punct:]]' collate builtin_c_utf8; -- symbols are punctuation in posix
+ ?column? 
+----------
+ t
+(1 row)
+
+select 'a8a' ~ '[[:digit:]]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select '൧' !~ '\d' collate builtin_c_utf8; -- only 0-9 considered digits in posix
+ ?column? 
+----------
+ t
+(1 row)
+
+-- case mapping
+select 'xYz' ~* 'XyZ' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select 'xAb' ~* '[W-Y]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select 'xAb' !~* '[c-d]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+select 'Δ' ~* '[α-λ]' collate builtin_c_utf8;
+ ?column? 
+----------
+ t
+(1 row)
+
+DROP COLLATION builtin_c_utf8;
diff --git a/src/test/regress/expected/collate.utf8_1.out b/src/test/regress/expected/collate.utf8_1.out
new file mode 100644
index 0000000000..e73fdf50c3
--- /dev/null
+++ b/src/test/regress/expected/collate.utf8_1.out
@@ -0,0 +1,8 @@
+/*
+ * This test is for collations and character operations when using the
+ * builtin provider with the C.UTF-8 locale.
+ */
+/* skip test if not UTF8 server encoding */
+SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
+\if :skip_test
+\quit
diff --git a/src/test/regress/parallel_schedule b/src/test/regress/parallel_schedule
index f0987ff537..292bc54932 100644
--- a/src/test/regress/parallel_schedule
+++ b/src/test/regress/parallel_schedule
@@ -78,9 +78,9 @@ test: brin_bloom brin_multi
 # psql depends on create_am
 # amutils depends on geometry, create_index_spgist, hash_index, brin
 # ----------
-test: create_table_like alter_generic alter_operator misc async dbsize merge misc_functions sysviews tsrf tid tidscan tidrangescan collate.icu.utf8 incremental_sort create_role
+test: create_table_like alter_generic alter_operator misc async dbsize merge misc_functions sysviews tsrf tid tidscan tidrangescan collate.utf8 collate.icu.utf8 incremental_sort create_role
 
-# collate.*.utf8 tests cannot be run in parallel with each other
+# collate.linux.utf8 and collate.icu.utf8 tests cannot be run in parallel with each other
 test: rules psql psql_crosstab amutils stats_ext collate.linux.utf8 collate.windows.win1252
 
 # ----------
diff --git a/src/test/regress/sql/collate.sql b/src/test/regress/sql/collate.sql
index c3d40fc195..01d5c69fe4 100644
--- a/src/test/regress/sql/collate.sql
+++ b/src/test/regress/sql/collate.sql
@@ -244,6 +244,16 @@ EXPLAIN (COSTS OFF)
 
 -- CREATE/DROP COLLATION
 
+CREATE COLLATION builtin_c ( PROVIDER = builtin, LOCALE = "C" );
+CREATE COLLATION builtin_posix ( PROVIDER = builtin, LOCALE = "POSIX" );
+
+SELECT b FROM collate_test1 ORDER BY b COLLATE builtin_c;
+
+CREATE COLLATION builtin2 ( PROVIDER = builtin ); -- fails
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "en_US" ); -- fails
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LC_CTYPE = "C", LC_COLLATE = "C" ); -- fails
+CREATE COLLATION builtin2 ( PROVIDER = builtin, LOCALE = "POSIX", LC_CTYPE = "POSIX" ); -- fails
+
 CREATE COLLATION mycoll1 FROM "C";
 CREATE COLLATION mycoll2 ( LC_COLLATE = "POSIX", LC_CTYPE = "POSIX" );
 CREATE COLLATION mycoll3 FROM "default";  -- intentionally unsupported
diff --git a/src/test/regress/sql/collate.utf8.sql b/src/test/regress/sql/collate.utf8.sql
new file mode 100644
index 0000000000..dadd8f8032
--- /dev/null
+++ b/src/test/regress/sql/collate.utf8.sql
@@ -0,0 +1,47 @@
+/*
+ * This test is for collations and character operations when using the
+ * builtin provider with the C.UTF-8 locale.
+ */
+
+/* skip test if not UTF8 server encoding */
+SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+
+SET client_encoding TO UTF8;
+
+--
+-- Test builtin provider with C.UTF-8 locale.
+--
+CREATE COLLATION builtin_c_utf8 (PROVIDER = builtin, LOCALE = 'C.UTF-8');
+
+CREATE TABLE builtin_test (
+  t TEXT COLLATE builtin_c_utf8
+);
+INSERT INTO builtin_test VALUES
+  ('abc DEF'),
+  ('DŽxxx Džxxx džxxx'),
+  ('xxxDŽ xxxDž xxxdž');
+
+SELECT t, lower(t), initcap(t), upper(t) FROM builtin_test;
+
+DROP TABLE builtin_test;
+
+-- character classes
+
+select 'xyz' ~ '[[:alnum:]]' collate builtin_c_utf8;
+select 'xyz' !~ '[[:upper:]]' collate builtin_c_utf8;
+select '@' !~ '[[:alnum:]]' collate builtin_c_utf8;
+select '@' ~ '[[:punct:]]' collate builtin_c_utf8; -- symbols are punctuation in posix
+select 'a8a' ~ '[[:digit:]]' collate builtin_c_utf8;
+select '൧' !~ '\d' collate builtin_c_utf8; -- only 0-9 considered digits in posix
+
+-- case mapping
+
+select 'xYz' ~* 'XyZ' collate builtin_c_utf8;
+select 'xAb' ~* '[W-Y]' collate builtin_c_utf8;
+select 'xAb' !~* '[c-d]' collate builtin_c_utf8;
+select 'Δ' ~* '[α-λ]' collate builtin_c_utf8;
+
+DROP COLLATION builtin_c_utf8;
-- 
2.34.1