diff --git a/Lib/re/_compiler.py b/Lib/re/_compiler.py
index c2ca8e25abe34d..0c76cef5f600be 100644
--- a/Lib/re/_compiler.py
+++ b/Lib/re/_compiler.py
@@ -266,7 +266,9 @@ def _optimize_charset(charset, iscased=None, fixup=None, fixes=None):
                     else:
                         charmap[av] = 1
                 elif op is RANGE:
-                    r = range(av[0], av[1]+1)
+                    start, end = av
+                    end += 1
+                    r = range(start, end)
                     if fixup: # IGNORECASE and not LOCALE
                         if fixes:
                             for i in map(fixup, r):
@@ -280,8 +282,10 @@ def _optimize_charset(charset, iscased=None, fixup=None, fixes=None):
                         if not hascased:
                             hascased = any(map(iscased, r))
                     else:
-                        for i in r:
-                            charmap[i] = 1
+                        if end > len(charmap):
+                            # Trigger the IndexError growth path below.
+                            raise IndexError
+                        charmap[start:end] = b'\x01' * (end - start)
                 elif op is NEGATE:
                     out.append((op, av))
                 elif op is CATEGORY and tail and (CATEGORY, CH_NEGATE[av]) in tail:
diff --git a/Misc/NEWS.d/next/Library/2026-05-05-18-39-52.gh-issue-149427.0ggedQ.rst b/Misc/NEWS.d/next/Library/2026-05-05-18-39-52.gh-issue-149427.0ggedQ.rst
new file mode 100644
index 00000000000000..38d6c21f1ef988
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-05-05-18-39-52.gh-issue-149427.0ggedQ.rst
@@ -0,0 +1,3 @@
+Speed up :func:`re.compile` of patterns with character ranges by replacing
+the per-byte loop in :mod:`!re._compiler` with a single bytearray slice
+fill.