diff --git a/Lib/re/_compiler.py b/Lib/re/_compiler.py
index c2ca8e25abe34d..ec1f9c3788d89e 100644
--- a/Lib/re/_compiler.py
+++ b/Lib/re/_compiler.py
@@ -280,8 +280,11 @@ 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
+                        end = av[1] + 1
+                        if end > len(charmap):
+                            # Trigger the IndexError growth path below.
+                            raise IndexError
+                        charmap[av[0]:end] = b'\x01' * (end - av[0])
                 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.