Handle Surrogates If Read

It is probably very anal of me to do this. Excel does it. I can't see it happening in the wild.

src/PhpSpreadsheet/Shared/StringHelper.php

@@ -305,7 +305,7 @@ class StringHelper
      */
     public static function controlCharacterOOXML2PHP(string $textValue): string
     {
-        return Preg::replaceCallback('/_x[0-9A-F]{4}_/', self::toOutChar(...), $textValue);
+        return Preg::replaceCallback('/_x[0-9A-F]{4}_(_xD[CDEF][0-9A-F]{2}_)?/', self::toOutChar(...), $textValue);
     }
 
     private static function toHexVal(string $char): int
@@ -322,12 +322,27 @@ class StringHelper
     {
         /** @var string */
         $chars = $match[0];
-        $t = ((self::toHexVal($chars[2]) << 12)
+        $h = ((self::toHexVal($chars[2]) << 12)
             | (self::toHexVal($chars[3]) << 8)
             | (self::toHexVal($chars[4]) << 4)
             | (self::toHexVal($chars[5])));
+        if (strlen($chars) === 7) { // no low surrogate
+            if ($chars[2] === 'D' && in_array($chars[3], ['8', '9', 'A', 'B', 'C', 'D', 'E', 'F'], true)) {
+                return mb_chr(0xFFFD, 'UTF-8');
+            }
 
-        return mb_chr($t, 'UTF-8');
+            return mb_chr($h, 'UTF-8');
+        }
+        if ($chars[2] !== 'D' || !in_array($chars[3], ['8', '9', 'A', 'B'], true)) {
+            return mb_chr($h, 'UTF-8') . mb_chr(0xFFFD, 'UTF-8');
+        }
+        $l = ((self::toHexVal($chars[9]) << 12)
+            | (self::toHexVal($chars[10]) << 8)
+            | (self::toHexVal($chars[11]) << 4)
+            | (self::toHexVal($chars[12])));
+        $result = 0x10000 + ($h - 0xD800) * 0x400 + ($l - 0xDC00);
+
+        return mb_chr($result, 'UTF-8');
     }
 
     /**

tests/PhpSpreadsheetTests/Reader/Xlsx/UnderscoreTest.php

@@ -71,7 +71,7 @@ class UnderscoreTest extends AbstractFunctional
         if ($data === false) {
             self::fail('Unable to read file');
         } else {
-            self::assertStringContainsString('count="4"', $data);
+            self::assertStringContainsString('count="7"', $data);
             self::assertStringContainsString(
                 "<t>line_x000D_\nwith_x000D_\nbreaks</t>",
                 $data
@@ -82,6 +82,9 @@ class UnderscoreTest extends AbstractFunctional
                 $data
             );
             self::assertStringContainsString('<t>_xC1EF_</t>', $data);
+            self::assertStringContainsString('<t>_xD801__xDC05_</t>', $data);
+            self::assertStringContainsString('<t>_xD801__x0038_</t>', $data);
+            self::assertStringContainsString('<t>_x0039__xDC05_</t>', $data);
         }
     }
 
@@ -102,6 +105,12 @@ class UnderscoreTest extends AbstractFunctional
         self::assertSame($expected, $sheet->getCell('A3')->getValue());
         $expected = '쇯';
         self::assertSame($expected, $sheet->getCell('A4')->getValue());
+        $expected = '𐐅';
+        self::assertSame($expected, $sheet->getCell('A5')->getValue(), 'outside BMP');
+        $expected = '�8';
+        self::assertSame($expected, $sheet->getCell('A6')->getValue(), 'high surrogate without low');
+        $expected = '9�';
+        self::assertSame($expected, $sheet->getCell('A7')->getValue(), 'low surrogate without high');
     }
 
     public function testX000dNotPreserved(): void