Index: formatting.php
===================================================================
--- formatting.php	(revision 29522)
+++ formatting.php	(working copy)
@@ -695,48 +695,110 @@
 /**
  * Checks for invalid UTF8 in a string.
  *
+ * Could change the ini_setting mbstring.substitute_character to 'none' without restoring.
+ *
  * @since 2.8.0
  *
  * @param string $string The text which is to be checked.
  * @param boolean $strip Optional. Whether to attempt to strip out invalid UTF8. Default is false.
- * @return string The checked text.
+ * @return string If the string is valid UTF-8 or the blog_charset is not UTF-8, the string is returned unmodified. Otherwise, an empty string is returned, or optionally the string stripped of invalid chars.
  */
 function wp_check_invalid_utf8( $string, $strip = false ) {
 	$string = (string) $string;
 
-	if ( 0 === strlen( $string ) ) {
+	// if string length is 0 (faster than strlen) return empty
+	if ( ! isset( $string[0] ) ) {
 		return '';
 	}
 
-	// Store the site charset as a static to avoid multiple calls to get_option()
+	// Store the site charset
 	static $is_utf8;
-	if ( !isset( $is_utf8 ) ) {
-		$is_utf8 = in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ) );
+
+	// if first time this function is called, save boolean if utf-8 enabled or not static
+	if ( ! isset( $is_utf8 ) ) {
+		$is_utf8 = ( in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ) ) );
 	}
-	if ( !$is_utf8 ) {
+
+	// if utf not used return the string
+	if ( ! $is_utf8 ) {
 		return $string;
 	}
 
+	// Whether pcre is enabled, and whether pcre needs to use the (*UTF8) trick
+	static $pcre_utf8;
+
 	// Check for support for utf8 in the installed PCRE library once and store the result in a static
-	static $utf8_pcre;
-	if ( !isset( $utf8_pcre ) ) {
-		$utf8_pcre = @preg_match( '/^./u', 'a' );
+	if ( ! isset( $pcre_utf8 ) ) {
+		$pcre_utf8 = ( @preg_match( '//u', '' ) !== false );
 	}
-	// We can't demand utf8 in the PCRE installation, so just return the string in those cases
-	if ( !$utf8_pcre ) {
+
+	// If pcre_utf /u modifier allowed and the utf is valid, return string
+	if ( $pcre_utf8 && @preg_match( '//u', $string ) !== false ) {
 		return $string;
 	}
 
-	// preg_match fails when it encounters invalid UTF8 in $string
-	if ( 1 === @preg_match( '/^./us', $string ) ) {
-		return $string;
+	// just means that the /u modifier is disallowed, so try the pattern option
+	if ( ! $pcre_utf8 ) {
+
+		// Use the pattern option for pcre and return string if valid.  Else use the regex
+		if ( @preg_match( '/(*UTF8)/', $string ) !== false ) {
+			return $string;
+		} else {
+
+			// pcre was compiled explicitly to forbid the support of UTF.  So use a regex to check (third times the charm)
+			$pattern = '/(
+				[\xC0-\xC1] # Invalid UTF-8 Bytes
+				| [\xF5-\xFF] # Invalid UTF-8 Bytes
+				| \xE0[\x80-\x9F] # Overlong encoding of prior code point
+				| \xF0[\x80-\x8F] # Overlong encoding of prior code point
+				| [\xC2-\xDF](?![\x80-\xBF]) # Invalid UTF-8 Sequence Start
+				| [\xE0-\xEF](?![\x80-\xBF]{2}) # Invalid UTF-8 Sequence Start
+				| [\xF0-\xF4](?![\x80-\xBF]{3}) # Invalid UTF-8 Sequence Start
+				| (?<=[\x0-\x7F\xF5-\xFF])[\x80-\xBF] # Invalid UTF-8 Sequence Middle
+				| (?<![\xC2-\xDF]|[\xE0-\xEF]|[\xE0-\xEF][\x80-\xBF]|[\xF0-\xF4]|[\xF0-\xF4][\x80-\xBF]|[\xF0-\xF4][\x80-\xBF]{2})[\x80-\xBF] # Overlong Sequence
+				| (?<=[\xE0-\xEF])[\x80-\xBF](?![\x80-\xBF]) # Short 3 byte sequence
+				| (?<=[\xF0-\xF4])[\x80-\xBF](?![\x80-\xBF]{2}) # Short 4 byte sequence
+				| (?<=[\xF0-\xF4][\x80-\xBF])[\x80-\xBF](?![\x80-\xBF]) # Short 4 byte sequence (2)
+			)/x';
+
+			// if the utf is valid return the string
+			if ( @preg_match( $pattern , $string ) === 1 ) {
+				return $string;
+			}
+		}
 	}
 
 	// Attempt to strip the bad chars if requested (not recommended)
-	if ( $strip && function_exists( 'iconv' ) ) {
-		return iconv( 'utf-8', 'utf-8', $string );
+	if ( $strip ) {
+		// Whether inconv is available
+		static $iconv;
+
+		// Check once for support for iconv and save statically, if not set the mbstring.substitute_character ini
+		if ( ! isset( $iconv ) ) {
+			$iconv = function_exists( 'iconv' );
+
+			if ( ! $iconv ) {
+				// only create a static var if $iconv is not available since only then will the final if ( $mb_convert ) check utilize this var
+				static $mb_convert;
+
+				// if mb_convert_encoding is available, set the ini_setting once			
+				@ini_set( 'mbstring.substitute_character', 'none' );
+				$mb_convert = ( function_exists( 'mb_convert_encoding' ) && @ini_get( 'mbstring.substitute_character' ) === 'none' );
+			}
+		}
+
+		// use iconv if exists
+		if ( $iconv ) {
+			return @iconv( 'utf-8', 'utf-8//ignore', $string );
+		}
+
+		// otherwise try to use mb_convert_encoding, setting the substitue_character to none to mimic strip
+		if ( $mb_convert ) {
+			return @mb_convert_encoding( $string, 'utf-8', 'utf-8' );
+		}
 	}
 
+	// default to returning empty string, meaning invalid utf was found
 	return '';
 }