[Fix] Str::Mask() for consecutively repeating chars (laravel#42295)

- Add some additional test cases to verify behaviour for
  more than 2 repeating characters.
- Add some cases for testing the masking from the start of
  the string, before the start of the string and end of string.

(cherry picked from commit a917528)

src/Illuminate/Support/Str.php

@@ -437,10 +437,18 @@ public static function mask($string, $character, $index, $length = null, $encodi
             return $string;
         }
 
-        $start = mb_substr($string, 0, mb_strpos($string, $segment, 0, $encoding), $encoding);
-        $end = mb_substr($string, mb_strpos($string, $segment, 0, $encoding) + mb_strlen($segment, $encoding));
+        $strlen = mb_strlen($string, $encoding);
+        $startIndex = $index;
 
-        return $start.str_repeat(mb_substr($character, 0, 1, $encoding), mb_strlen($segment, $encoding)).$end;
+        if ($index < 0) {
+            $startIndex = $index < -$strlen ? 0 : $strlen + $index;
+        }
+
+        $start = mb_substr($string, 0, $startIndex, $encoding);
+        $segmentLen = mb_strlen($segment, $encoding);
+        $end = mb_substr($string, $startIndex + $segmentLen);
+
+        return $start.str_repeat(mb_substr($character, 0, 1, $encoding), $segmentLen).$end;
     }
 
     /**

tests/Support/SupportStrTest.php

@@ -508,6 +508,19 @@ public function testMask()
 
         $this->assertSame('这是一***', Str::mask('这是一段中文', '*', 3));
         $this->assertSame('**一段中文', Str::mask('这是一段中文', '*', 0, 2));
+
+        $this->assertSame('ma*[email protected]', Str::mask('[email protected]', '*', 2, 1));
+        $this->assertSame('ma***email.com', Str::mask('[email protected]', '*', 2, 3));
+        $this->assertSame('ma************', Str::mask('[email protected]', '*', 2));
+
+        $this->assertSame('mari*@email.com', Str::mask('[email protected]', '*', 4, 1));
+        $this->assertSame('tamar*@email.com', Str::mask('[email protected]', '*', 5, 1));
+
+        $this->assertSame('*[email protected]', Str::mask('[email protected]', '*', 0, 1));
+        $this->assertSame('[email protected]*', Str::mask('[email protected]', '*', -1, 1));
+        $this->assertSame('[email protected]*', Str::mask('[email protected]', '*', -1));
+        $this->assertSame('***************', Str::mask('[email protected]', '*', -15));
+        $this->assertSame('***************', Str::mask('[email protected]', '*', 0));
     }
 
     public function testMatch()