diff -u b/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
--- b/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -412,8 +412,8 @@
       // Find the starting byte offset.
       $bytes = 0;
       if ($start > 0) {
-        // Count all the continuation bytes from the start until we have found
-        // $start characters or the end of the string.
+        // Count all the characters except continuation bytes from the start
+        // until we have found $start characters or the end of the string.
         $bytes = -1; $chars = -1;
         while ($bytes < $strlen - 1 && $chars < $start) {
           $bytes++;
@@ -424,8 +424,8 @@
         }
       }
       elseif ($start < 0) {
-        // Count all the continuation bytes from the end until we have found
-        // abs($start) characters.
+        // Count all the characters except continuation bytes from the end
+        // until we have found abs($start) characters.
         $start = abs($start);
         $bytes = $strlen; $chars = 0;
         while ($bytes > 0 && $chars < $start) {
@@ -443,9 +443,9 @@
         $iend = $strlen;
       }
       elseif ($length > 0) {
-        // Count all the continuation bytes from the starting index until we have
-        // found $length characters or reached the end of the string, then
-        // backtrace one byte.
+        // Count all the characters except continuation bytes from the starting
+        // index until we have found $length characters or reached the end of
+        // the string, then backtrace one byte.
         $iend = $istart - 1;
         $chars = -1;
         $last_real = FALSE;
@@ -458,15 +458,15 @@
             $last_real = TRUE;
           }
         }
-        // Backtrace one byte if the last character we found was a real character
-        // and we don't need it.
+        // Backtrace one byte if the last character we found was a real
+        // character and we don't need it.
         if ($last_real && $chars >= $length) {
           $iend--;
         }
       }
       elseif ($length < 0) {
-        // Count all the continuation bytes from the end until we have found
-        // abs($start) characters, then backtrace one byte.
+        // Count all the characters except continuation bytes from the end
+        // until we have found abs($start) characters, then backtrace one byte.
         $length = abs($length);
         $iend = $strlen; $chars = 0;
         while ($iend > 0 && $chars < $length) {
@@ -718,6 +718,10 @@
       return mb_strpos($haystack, $needle, $offset);
     }
     else {
+      // Remove Unicode continuation characters, to be compatible with
+      // Unicode::strlen() and Unicode::substr().
+      $haystack = preg_replace("/[\x80-\xBF]/", '', $haystack);
+      $needle = preg_replace("/[\x80-\xBF]/", '', $needle);
       return strpos($haystack, $needle, $offset);
     }
   }
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
@@ -291,6 +291,7 @@ public function providerStrlen() {
     return array(
       array('tHe QUIcK bRoWn', 15),
       array('ÜBER-åwesome', 12),
+      array('以呂波耳・ほへとち。リヌルヲ。', 15),
     );
   }
 
@@ -529,4 +530,46 @@ public function providerTestConvertToUtf8() {
     );
   }
 
+  /**
+   * Tests multibyte strpos.
+   *
+   * @dataProvider providerStrpos
+   * @covers ::strpos
+   */
+  public function testStrpos($haystack, $needle, $offset, $expected) {
+    // Run through multibyte code path.
+    Unicode::setStatus(Unicode::STATUS_MULTIBYTE);
+    $this->assertEquals($expected, Unicode::strpos($haystack, $needle, $offset));
+    // Run through singlebyte code path.
+    Unicode::setStatus(Unicode::STATUS_SINGLEBYTE);
+    $this->assertEquals($expected, Unicode::strpos($haystack, $needle, $offset));
+  }
+
+  /**
+   * Data provider for testStrpos().
+   *
+   * @see testStrpos()
+   *
+   * @return array
+   *   An array containing:
+   *     - The haystack string to be searched in.
+   *     - The needle string to search for.
+   *     - The offset integer to start at.
+   *     - The expected integer/FALSE result.
+   */
+  public function providerStrpos() {
+    return array(
+      array('frànçAIS is über-åwesome', 'frànçAIS is über-åwesome', 0, 0),
+      array('frànçAIS is über-åwesome', 'rànçAIS is über-åwesome', 0, 1),
+      array('frànçAIS is über-åwesome', 'not in string', 0, FALSE),
+      array('frànçAIS is über-åwesome', 'r', 0, 1),
+      array('frànçAIS is über-åwesome', 'nçAIS', 0, 3),
+      array('frànçAIS is über-åwesome', 'nçAIS', 2, 3),
+      array('frànçAIS is über-åwesome', 'nçAIS', 3, 3),
+      array('以呂波耳・ほへとち。リヌルヲ。', '波耳', 0, 2),
+      array('以呂波耳・ほへとち。リヌルヲ。', '波耳', 1, 2),
+      array('以呂波耳・ほへとち。リヌルヲ。', '波耳', 2, 2),
+    );
+  }
+
 }