diff --git a/Lib/sre_compile.py b/Lib/sre_compile.py
index d6dc60c73ed..e08ec661aaa 100644
--- a/Lib/sre_compile.py
+++ b/Lib/sre_compile.py
@@ -339,7 +339,7 @@ def _optimize_unicode(charset, fixup):
     else:
         code = 'I'
     # Convert block indices to byte array of 256 bytes
-    mapping = array.array('b', mapping).tobytes()
+    mapping = array.array('B', mapping).tobytes()
     # Convert byte array to word array
     mapping = array.array(code, mapping)
     assert mapping.itemsize == _sre.CODESIZE
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 841d3a38c53..5e685850653 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -482,6 +482,9 @@ class ReTests(unittest.TestCase):
                                   "\u2222").group(1), "\u2222")
         self.assertEqual(re.match("([\u2222\u2223])",
                                   "\u2222", re.UNICODE).group(1), "\u2222")
+        r = '[%s]' % ''.join(map(chr, range(256, 2**16, 255)))
+        self.assertEqual(re.match(r,
+                                  "\uff01", re.UNICODE).group(), "\uff01")
 
     def test_big_codesize(self):
         # Issue #1160
diff --git a/Misc/NEWS b/Misc/NEWS
index 9a3e08a856f..26e418b2a8d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -19,6 +19,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #19327: Fixed the working of regular expressions with too big charset.
+
 - Issue #17400: New 'is_global' attribute for ipaddress to tell if an address
   is allocated by IANA for global or private networks.
 
diff --git a/Modules/_sre.c b/Modules/_sre.c
index 84330ef7152..dd9d6645d70 100644
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -447,7 +447,7 @@ SRE_CHARSET(SRE_CODE* set, SRE_CODE ch)
             count = *(set++);
 
             if (sizeof(SRE_CODE) == 2) {
-                block = ((char*)set)[ch >> 8];
+                block = ((unsigned char*)set)[ch >> 8];
                 set += 128;
                 if (set[block*16 + ((ch & 255)>>4)] & (1 << (ch & 15)))
                     return ok;
@@ -457,7 +457,7 @@ SRE_CHARSET(SRE_CODE* set, SRE_CODE ch)
                 /* !(c & ~N) == (c < N+1) for any unsigned c, this avoids
                  * warnings when c's type supports only numbers < N+1 */
                 if (!(ch & ~65535))
-                    block = ((char*)set)[ch >> 8];
+                    block = ((unsigned char*)set)[ch >> 8];
                 else
                     block = -1;
                 set += 64;