diff --git a/src/wp-includes/l10n/class-wp-translation-file.php b/src/wp-includes/l10n/class-wp-translation-file.php
index 93842bec10..e6dbdb85be 100644
--- a/src/wp-includes/l10n/class-wp-translation-file.php
+++ b/src/wp-includes/l10n/class-wp-translation-file.php
@@ -207,7 +207,7 @@ abstract class WP_Translation_File {
 	}
 
 	/**
-	 * Returns the plural form for a count.
+	 * Returns the plural form for a given number.
 	 *
 	 * @since 6.5.0
 	 *
@@ -219,9 +219,9 @@ abstract class WP_Translation_File {
 			$this->parse_file();
 		}
 
-		// In case a plural form is specified as a header, but no function included, build one.
 		if ( null === $this->plural_forms && isset( $this->headers['plural-forms'] ) ) {
-			$this->plural_forms = $this->make_plural_form_function( $this->headers['plural-forms'] );
+			$expression         = $this->get_plural_expression_from_header( $this->headers['plural-forms'] );
+			$this->plural_forms = $this->make_plural_form_function( $expression );
 		}
 
 		if ( is_callable( $this->plural_forms ) ) {
@@ -231,6 +231,7 @@ abstract class WP_Translation_File {
 			 * @var int $result Plural form.
 			 */
 			$result = call_user_func( $this->plural_forms, $number );
+
 			return $result;
 		}
 
@@ -238,6 +239,22 @@ abstract class WP_Translation_File {
 		return ( 1 === $number ? 0 : 1 );
 	}
 
+	/**
+	 * Returns the plural forms expression as a tuple.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @param string $header Plural-Forms header string.
+	 * @return string Plural forms expression.
+	 */
+	protected function get_plural_expression_from_header( $header ) {
+		if ( preg_match( '/^\s*nplurals\s*=\s*(\d+)\s*;\s+plural\s*=\s*(.+)$/', $header, $matches ) ) {
+			return trim( $matches[2] );
+		}
+
+		return 'n != 1';
+	}
+
 	/**
 	 * Makes a function, which will return the right translation index, according to the
 	 * plural forms header.
@@ -247,7 +264,7 @@ abstract class WP_Translation_File {
 	 * @param string $expression Plural form expression.
 	 * @return callable(int $num): int Plural forms function.
 	 */
-	public function make_plural_form_function( string $expression ): callable {
+	protected function make_plural_form_function( string $expression ): callable {
 		try {
 			$handler = new Plural_Forms( rtrim( $expression, ';' ) );
 			return array( $handler, 'get' );
diff --git a/tests/phpunit/data/l10n/plural-complex.mo b/tests/phpunit/data/l10n/plural-complex.mo
new file mode 100644
index 0000000000..9cb7b2820e
Binary files /dev/null and b/tests/phpunit/data/l10n/plural-complex.mo differ
diff --git a/tests/phpunit/data/l10n/plural-complex.php b/tests/phpunit/data/l10n/plural-complex.php
new file mode 100644
index 0000000000..b214214f2a
--- /dev/null
+++ b/tests/phpunit/data/l10n/plural-complex.php
@@ -0,0 +1,2 @@
+<?php
+return ['x-generator'=>'GlotPress/4.0.0-beta.2','translation-revision-date'=>'2024-01-18 05:40:05+0000','plural-forms'=>'nplurals=4; plural=(n % 100 == 1) ? 0 : ((n % 100 == 2) ? 1 : ((n % 100 == 3 || n % 100 == 4) ? 2 : 3));','project-id-version'=>'WordPress - 6.4.x - Development','language'=>'sl_SI','messages'=>['%s update available'=>'%s razpoložljiva posodobitev' . "\0" . '%s razpoložljivi posodobitvi' . "\0" . '%s razpoložljive posodobitve' . "\0" . '%s razpoložljivih posodobitev']];
\ No newline at end of file
diff --git a/tests/phpunit/tests/l10n/wpTranslations.php b/tests/phpunit/tests/l10n/wpTranslations.php
index 3879a570e3..70abc4fd30 100644
--- a/tests/phpunit/tests/l10n/wpTranslations.php
+++ b/tests/phpunit/tests/l10n/wpTranslations.php
@@ -219,6 +219,52 @@ class WP_Translations_Tests extends WP_UnitTestCase {
 		$this->assertTrue( $unload_successful, 'Text domain not successfully unloaded' );
 	}
 
+	/**
+	 * @covers ::translate_plural
+	 * @covers WP_Translation_File::get_plural_form
+	 */
+	public function test_translate_plural_complex() {
+		load_textdomain( 'wp-tests-domain', DIR_TESTDATA . '/l10n/plural-complex.mo' );
+
+		$this->assertSame( '%s razpoložljiva posodobitev', _n( '%s update available', '%s updates available', 101, 'wp-tests-domain' ) ); // 1, 101, 201
+		$this->assertSame( '%s razpoložljivi posodobitvi', _n( '%s update available', '%s updates available', 102, 'wp-tests-domain' ) ); // 2, 102, 202
+		$this->assertSame( '%s razpoložljive posodobitve', _n( '%s update available', '%s updates available', 103, 'wp-tests-domain' ) ); // 3, 4, 103
+		$this->assertSame( '%s razpoložljivih posodobitev', _n( '%s update available', '%s updates available', 5, 'wp-tests-domain' ) ); // 0, 5, 6
+	}
+
+	/**
+	 * @covers ::translate_plural
+	 * @covers WP_Translation_File::get_plural_form
+	 */
+	public function test_translate_plural_complex_php() {
+		load_textdomain( 'wp-tests-domain', DIR_TESTDATA . '/l10n/plural-complex.php' );
+
+		$this->assertSame( '%s razpoložljiva posodobitev', _n( '%s update available', '%s updates available', 101, 'wp-tests-domain' ) ); // 1, 101, 201
+		$this->assertSame( '%s razpoložljivi posodobitvi', _n( '%s update available', '%s updates available', 102, 'wp-tests-domain' ) ); // 2, 102, 202
+		$this->assertSame( '%s razpoložljive posodobitve', _n( '%s update available', '%s updates available', 103, 'wp-tests-domain' ) ); // 3, 4, 103
+		$this->assertSame( '%s razpoložljivih posodobitev', _n( '%s update available', '%s updates available', 5, 'wp-tests-domain' ) ); // 0, 5, 6
+	}
+
+	/**
+	 * @covers WP_Translation_File::get_plural_form
+	 */
+	public function test_get_plural_form() {
+		$moe = WP_Translation_File::create( DIR_TESTDATA . '/l10n/plural-complex.mo' );
+
+		$this->assertSame( 0, $moe->get_plural_form( 1 ) );
+		$this->assertSame( 0, $moe->get_plural_form( 101 ) );
+		$this->assertSame( 0, $moe->get_plural_form( 201 ) );
+		$this->assertSame( 1, $moe->get_plural_form( 2 ) );
+		$this->assertSame( 1, $moe->get_plural_form( 102 ) );
+		$this->assertSame( 1, $moe->get_plural_form( 202 ) );
+		$this->assertSame( 2, $moe->get_plural_form( 3 ) );
+		$this->assertSame( 2, $moe->get_plural_form( 4 ) );
+		$this->assertSame( 2, $moe->get_plural_form( 103 ) );
+		$this->assertSame( 3, $moe->get_plural_form( 0 ) );
+		$this->assertSame( 3, $moe->get_plural_form( 5 ) );
+		$this->assertSame( 3, $moe->get_plural_form( 6 ) );
+	}
+
 	/**
 	 * @covers ::translate_plural
 	 */