diff --git a/src/wp-includes/SimplePie/src/Misc.php b/src/wp-includes/SimplePie/src/Misc.php
index cc5db8aea39e0..dc88f9067c912 100644
--- a/src/wp-includes/SimplePie/src/Misc.php
+++ b/src/wp-includes/SimplePie/src/Misc.php
@@ -1875,7 +1875,11 @@ public static function atom_10_content_construct_type($attribs)
 
     public static function is_isegment_nz_nc($string)
     {
-        return (bool) preg_match('/^([A-Za-z0-9\-._~\x{A0}-\x{D7FF}\x{F900}-\x{FDCF}\x{FDF0}-\x{FFEF}\x{10000}-\x{1FFFD}\x{20000}-\x{2FFFD}\x{30000}-\x{3FFFD}\x{40000}-\x{4FFFD}\x{50000}-\x{5FFFD}\x{60000}-\x{6FFFD}\x{70000}-\x{7FFFD}\x{80000}-\x{8FFFD}\x{90000}-\x{9FFFD}\x{A0000}-\x{AFFFD}\x{B0000}-\x{BFFFD}\x{C0000}-\x{CFFFD}\x{D0000}-\x{DFFFD}\x{E1000}-\x{EFFFD}!$&\'()*+,;=@]|(%[0-9ABCDEF]{2}))+$/u', $string);
+        if ( _wp_can_use_pcre_u() ) {
+            return (bool) preg_match('/^([A-Za-z0-9\-._~\x{A0}-\x{D7FF}\x{F900}-\x{FDCF}\x{FDF0}-\x{FFEF}\x{10000}-\x{1FFFD}\x{20000}-\x{2FFFD}\x{30000}-\x{3FFFD}\x{40000}-\x{4FFFD}\x{50000}-\x{5FFFD}\x{60000}-\x{6FFFD}\x{70000}-\x{7FFFD}\x{80000}-\x{8FFFD}\x{90000}-\x{9FFFD}\x{A0000}-\x{AFFFD}\x{B0000}-\x{BFFFD}\x{C0000}-\x{CFFFD}\x{D0000}-\x{DFFFD}\x{E1000}-\x{EFFFD}!$&\'()*+,;=@]|(%[0-9ABCDEF]{2}))+$/u', $string);
+        } else {
+            return (bool) preg_match('/^([A-Za-z0-9\-._~!$&\'()*+,;=@]|(%[0-9ABCDEF]{2}))+$/', $string);
+        }
     }
 
     public static function space_separated_tokens($string)
diff --git a/src/wp-includes/class-wp-plugin-dependencies.php b/src/wp-includes/class-wp-plugin-dependencies.php
index ee5e999903fb7..24f8aae54fe89 100644
--- a/src/wp-includes/class-wp-plugin-dependencies.php
+++ b/src/wp-includes/class-wp-plugin-dependencies.php
@@ -603,8 +603,14 @@ protected static function sanitize_dependency_slugs( $slugs ) {
 			$slug = apply_filters( 'wp_plugin_dependencies_slug', $slug );
 
 			// Match to WordPress.org slug format.
-			if ( preg_match( '/^[a-z0-9]+(-[a-z0-9]+)*$/mu', $slug ) ) {
-				$sanitized_slugs[] = $slug;
+			if ( _wp_can_use_pcre_u() ) {
+				if ( preg_match( '/^[a-z0-9]+(-[a-z0-9]+)*$/mu', $slug ) ) {
+					$sanitized_slugs[] = $slug;
+				}
+			} else {
+				if ( preg_match( '/^[a-z0-9]+(-[a-z0-9]+)*$/m', $slug ) ) {
+					$sanitized_slugs[] = $slug;
+				}
 			}
 		}
 		$sanitized_slugs = array_unique( $sanitized_slugs );
diff --git a/src/wp-includes/class-wp-text-diff-renderer-inline.php b/src/wp-includes/class-wp-text-diff-renderer-inline.php
index aef1738f1f795..c957e27afe6e8 100644
--- a/src/wp-includes/class-wp-text-diff-renderer-inline.php
+++ b/src/wp-includes/class-wp-text-diff-renderer-inline.php
@@ -26,8 +26,35 @@ class WP_Text_Diff_Renderer_inline extends Text_Diff_Renderer_inline {
 	 */
 	public function _splitOnWords( $string, $newlineEscape = "\n" ) { // phpcs:ignore Universal.NamingConventions.NoReservedKeywordParameterNames.stringFound,WordPress.NamingConventions.ValidVariableName.VariableNotSnakeCase
 		$string = str_replace( "\0", '', $string );
-		$words  = preg_split( '/([^\w])/u', $string, -1, PREG_SPLIT_DELIM_CAPTURE );
-		$words  = str_replace( "\n", $newlineEscape, $words ); // phpcs:ignore WordPress.NamingConventions.ValidVariableName.VariableNotSnakeCase
+		if ( _wp_can_use_pcre_u() ) {
+			$words = preg_split( '/([^\w])/u', $string, -1, PREG_SPLIT_DELIM_CAPTURE );
+		} else {
+			if ( function_exists( 'mb_str_split' ) ) {
+				$chars = mb_str_split( $string, 1, 'UTF-8' );
+			} else {
+				$chars = str_split( $string );
+			}
+			$words        = array();
+			$current_word = '';
+
+			foreach ( $chars as $char ) {
+				// Simple heuristic: letters, numbers, underscore = word characters
+				if ( ctype_alnum( $char ) || '_' === $char || ord( $char ) > 127 ) {
+					$current_word .= $char;
+				} else {
+					if ( '' !== $current_word ) {
+						$words[]      = $current_word;
+						$current_word = '';
+					}
+					$words[] = $char; // Capture delimiter
+				}
+			}
+			if ( '' !== $current_word ) {
+				$words[] = $current_word;
+			}
+		}
+
+		$words = str_replace( "\n", $newlineEscape, $words ); // phpcs:ignore WordPress.NamingConventions.ValidVariableName.VariableNotSnakeCase
 		return $words;
 	}
 }
diff --git a/src/wp-includes/comment.php b/src/wp-includes/comment.php
index aabe9f60dbb4a..3a478ade54084 100644
--- a/src/wp-includes/comment.php
+++ b/src/wp-includes/comment.php
@@ -97,7 +97,11 @@ function check_comment( $author, $email, $url, $comment, $user_ip, $user_agent,
 			 * Check the comment fields for moderation keywords. If any are found,
 			 * fail the check for the given field by returning false.
 			 */
-			$pattern = "#$word#iu";
+			if ( _wp_can_use_pcre_u() ) {
+				$pattern = "#$word#iu";
+			} else {
+				$pattern = "#$word#i";
+			}
 			if ( preg_match( $pattern, $author ) ) {
 				return false;
 			}
@@ -1415,7 +1419,11 @@ function wp_check_comment_disallowed_list( $author, $email, $url, $comment, $use
 		// Do some escaping magic so that '#' chars in the spam words don't break things:
 		$word = preg_quote( $word, '#' );
 
-		$pattern = "#$word#iu";
+		if ( _wp_can_use_pcre_u() ) {
+			$pattern = "#$word#iu";
+		} else {
+			$pattern = "#$word#i";
+		}
 		if ( preg_match( $pattern, $author )
 			|| preg_match( $pattern, $email )
 			|| preg_match( $pattern, $url )
diff --git a/src/wp-includes/formatting.php b/src/wp-includes/formatting.php
index 61450afb9e3ea..d6bb12a5d204e 100644
--- a/src/wp-includes/formatting.php
+++ b/src/wp-includes/formatting.php
@@ -1048,7 +1048,7 @@ function _wp_is_valid_utf8_fallback( string $bytes ): bool {
 		// Valid two-byte code points.
 
 		if ( $b1 >= 0xC2 && $b1 <= 0xDF && $b2 >= 0x80 && $b2 <= 0xBF ) {
-			$i++;
+			++$i;
 			continue;
 		}
 
@@ -1289,19 +1289,12 @@ function wp_check_invalid_utf8( $text, $strip = false ) {
 		return $text;
 	}
 
-	// Check for support for utf8 in the installed PCRE library once and store the result in a static.
-	static $utf8_pcre = null;
-	if ( ! isset( $utf8_pcre ) ) {
-		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged
-		$utf8_pcre = @preg_match( '/^./u', 'a' );
-	}
-	// We can't demand utf8 in the PCRE installation, so just return the string in those cases.
-	if ( ! $utf8_pcre ) {
+	// Check for support for utf8 in the installed PCRE library.
+	if ( ! _wp_can_use_pcre_u() ) {
 		return $text;
 	}
 
-	// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- preg_match fails when it encounters invalid UTF8 in $text.
-	if ( 1 === @preg_match( '/^./us', $text ) ) {
+	if ( 1 === preg_match( '/^./us', $text ) ) {
 		return $text;
 	}
 
@@ -4233,7 +4226,15 @@ function wp_trim_words( $text, $num_words = 55, $more = null ) {
 
 	if ( str_starts_with( wp_get_word_count_type(), 'characters' ) && preg_match( '/^utf\-?8$/i', get_option( 'blog_charset' ) ) ) {
 		$text = trim( preg_replace( "/[\n\r\t ]+/", ' ', $text ), ' ' );
-		preg_match_all( '/./u', $text, $words_array );
+		if ( _wp_can_use_pcre_u() ) {
+			preg_match_all( '/./u', $text, $words_array );
+		} else {
+			if ( function_exists( 'mb_str_split' ) ) {
+				$words_array = array( mb_str_split( $text, 1, 'UTF-8' ) );
+			} else {
+				$words_array = array( str_split( $text ) );
+			}
+		}
 		$words_array = array_slice( $words_array[0], 0, $num_words + 1 );
 		$sep         = '';
 	} else {
diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index cd7531f093cca..0f44d661e57c2 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -360,12 +360,20 @@ function wp_maybe_decline_date( $date, $format = '' ) {
 			$decline = preg_match( '#[dj]\.? F#', $format );
 		} else {
 			// If the format is not passed, try to guess it from the date string.
-			$decline = preg_match( '#\b\d{1,2}\.? [^\d ]+\b#u', $date );
+			if ( _wp_can_use_pcre_u() ) {
+				$decline = preg_match( '#\b\d{1,2}\.? [^\d ]+\b#u', $date );
+			} else {
+				$decline = preg_match( '#\b\d{1,2}\.? [^\d ]+\b#', $date );
+			}
 		}
 
 		if ( $decline ) {
 			foreach ( $months as $key => $month ) {
-				$months[ $key ] = '# ' . preg_quote( $month, '#' ) . '\b#u';
+				if ( _wp_can_use_pcre_u() ) {
+					$months[ $key ] = '# ' . preg_quote( $month, '#' ) . '\b#u';
+				} else {
+					$months[ $key ] = '# ' . preg_quote( $month, '#' ) . '\b#';
+				}
 			}
 
 			foreach ( $months_genitive as $key => $month ) {
@@ -383,12 +391,20 @@ function wp_maybe_decline_date( $date, $format = '' ) {
 			$decline = preg_match( '#F [dj]#', $format );
 		} else {
 			// If the format is not passed, try to guess it from the date string.
-			$decline = preg_match( '#\b[^\d ]+ \d{1,2}(st|nd|rd|th)?\b#u', trim( $date ) );
+			if ( _wp_can_use_pcre_u() ) {
+				$decline = preg_match( '#\b[^\d ]+ \d{1,2}(st|nd|rd|th)?\b#u', trim( $date ) );
+			} else {
+				$decline = preg_match( '#\b[^\d ]+ \d{1,2}(st|nd|rd|th)?\b#', trim( $date ) );
+			}
 		}
 
 		if ( $decline ) {
 			foreach ( $months as $key => $month ) {
-				$months[ $key ] = '#\b' . preg_quote( $month, '#' ) . ' (\d{1,2})(st|nd|rd|th)?([-–]\d{1,2})?(st|nd|rd|th)?\b#u';
+				if ( _wp_can_use_pcre_u() ) {
+					$months[ $key ] = '#\b' . preg_quote( $month, '#' ) . ' (\d{1,2})(st|nd|rd|th)?([-–]\d{1,2})?(st|nd|rd|th)?\b#u';
+				} else {
+					$months[ $key ] = '#\b' . preg_quote( $month, '#' ) . ' (\d{1,2})(st|nd|rd|th)?([-–]\d{1,2})?(st|nd|rd|th)?\b#';
+				}
 			}
 
 			foreach ( $months_genitive as $key => $month ) {
diff --git a/src/wp-includes/link-template.php b/src/wp-includes/link-template.php
index 674e0a6fbf3f6..5980acc041c43 100644
--- a/src/wp-includes/link-template.php
+++ b/src/wp-includes/link-template.php
@@ -4564,7 +4564,33 @@ function get_avatar_data( $id_or_email, $args = null ) {
 		}
 
 		if ( ! empty( $name ) ) {
-			if ( preg_match( '/\p{Han}|\p{Hiragana}|\p{Katakana}|\p{Hangul}/u', $name ) || false === strpos( $name, ' ' ) ) {
+			$is_cjk = false;
+
+			if ( _wp_can_use_pcre_u() ) {
+				$is_cjk = preg_match( '/\p{Han}|\p{Hiragana}|\p{Katakana}|\p{Hangul}/u', $name );
+			} elseif ( class_exists( 'IntlChar' ) ) {
+				$first_char = mb_substr( $name, 0, 1, 'UTF-8' );
+				$codepoint  = IntlChar::ord( $first_char );
+
+				if ( null !== $codepoint ) {
+					$block      = IntlChar::getBlockCode( $codepoint );
+					$cjk_blocks = array(
+						IntlChar::BLOCK_CODE_CJK_UNIFIED_IDEOGRAPHS,
+						IntlChar::BLOCK_CODE_HANGUL_SYLLABLES,
+						IntlChar::BLOCK_CODE_HIRAGANA,
+						IntlChar::BLOCK_CODE_KATAKANA,
+						IntlChar::BLOCK_CODE_CJK_UNIFIED_IDEOGRAPHS_EXTENSION_A,
+						IntlChar::BLOCK_CODE_CJK_UNIFIED_IDEOGRAPHS_EXTENSION_B,
+						IntlChar::BLOCK_CODE_CJK_UNIFIED_IDEOGRAPHS_EXTENSION_C,
+						IntlChar::BLOCK_CODE_CJK_UNIFIED_IDEOGRAPHS_EXTENSION_D,
+						IntlChar::BLOCK_CODE_HANGUL_JAMO,
+						IntlChar::BLOCK_CODE_HANGUL_COMPATIBILITY_JAMO,
+					);
+					$is_cjk     = in_array( $block, $cjk_blocks, true );
+				}
+			}
+
+			if ( $is_cjk || false === strpos( $name, ' ' ) ) {
 				$initials = mb_substr( $name, 0, min( 2, mb_strlen( $name, 'UTF-8' ) ), 'UTF-8' );
 			} else {
 				$first    = mb_substr( $name, 0, 1, 'UTF-8' );
diff --git a/src/wp-includes/pomo/po.php b/src/wp-includes/pomo/po.php
index a4e3cab4ef17a..81a97b02b39ac 100644
--- a/src/wp-includes/pomo/po.php
+++ b/src/wp-includes/pomo/po.php
@@ -155,7 +155,13 @@ public static function unpoify( $input_string ) {
 			$unpoified             = '';
 			$previous_is_backslash = false;
 			foreach ( $lines as $line ) {
-				preg_match_all( '/./u', $line, $chars );
+				if ( _wp_can_use_pcre_u() ) {
+					preg_match_all( '/./u', $line, $chars );
+				} elseif ( function_exists( 'mb_str_split' ) ) {
+					$chars = array( mb_str_split( $line, 1, 'UTF-8' ) );
+				} else {
+					$chars = array( str_split( $line ) );
+				}
 				$chars = $chars[0];
 				foreach ( $chars as $char ) {
 					if ( ! $previous_is_backslash ) {
diff --git a/src/wp-includes/shortcodes.php b/src/wp-includes/shortcodes.php
index bddf63ada28a3..58af064037a4d 100644
--- a/src/wp-includes/shortcodes.php
+++ b/src/wp-includes/shortcodes.php
@@ -613,7 +613,11 @@ function get_shortcode_atts_regex() {
 function shortcode_parse_atts( $text ) {
 	$atts    = array();
 	$pattern = get_shortcode_atts_regex();
-	$text    = preg_replace( "/[\x{00a0}\x{200b}]+/u", ' ', $text );
+	if ( _wp_can_use_pcre_u() ) {
+		$text = preg_replace( "/[\x{00a0}\x{200b}]+/u", ' ', $text );
+	} else {
+		$text = str_replace( array( "\xc2\xa0", "\xe2\x80\x8b" ), ' ', $text );
+	}
 	if ( preg_match_all( $pattern, $text, $match, PREG_SET_ORDER ) ) {
 		foreach ( $match as $m ) {
 			if ( ! empty( $m[1] ) ) {