diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -130,6 +130,11 @@
   This ABI break only affects users that compile with ``-ffreestanding``, and only for ``atomic<T>`` where ``T``
   is a non-builtin type that could be lockfree on the platform. See https://llvm.org/D133377 for more details.
 
+- When building libc++ against newlib/picolibc, the type of ``regex_type_traits::char_class_type`` was changed to
+  uint16_t since all values of ``ctype_base::mask`` are taken. This is technically an ABI break, but including
+  ``<regex> `` has triggered a ``static_assert`` failure since libc++ 14, so it is unlikely that this causes a
+  real ABI break for existing users.
+
 Build System Changes
 --------------------
 - Support for ``libcxx``, ``libcxxabi`` and ``libunwind`` in ``LLVM_ENABLE_PROJECTS`` has officially
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -512,7 +512,8 @@
     static const mask punct  = _P;
     static const mask xdigit = _X | _N;
     static const mask blank  = _B;
-    static const mask __regex_word = 0x80;
+    // mask is already fully saturated, use a different type in regex_type_traits.
+    static const unsigned short __regex_word = 0x100;
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_PRINT
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_ALPHA
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_XDIGIT
@@ -551,7 +552,8 @@
 
     _LIBCPP_INLINE_VISIBILITY ctype_base() {}
 
-    static_assert((__regex_word & ~(space | print | cntrl | upper | lower | alpha | digit | punct | xdigit | blank)) == __regex_word,
+    static_assert((__regex_word & ~(std::make_unsigned<mask>::type)(space | print | cntrl | upper | lower | alpha |
+                                                                    digit | punct | xdigit | blank)) == __regex_word,
                   "__regex_word can't overlap other bits");
 };
 
diff --git a/libcxx/include/regex b/libcxx/include/regex
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -1026,7 +1026,7 @@
     typedef _CharT                  char_type;
     typedef basic_string<char_type> string_type;
     typedef locale                  locale_type;
-#ifdef __BIONIC__
+#if defined(__BIONIC__) || defined(_NEWLIB_VERSION)
     // Originally bionic's ctype_base used its own ctype masks because the
     // builtin ctype implementation wasn't in libc++ yet. Bionic's ctype mask
     // was only 8 bits wide and already saturated, so it used a wider type here
@@ -1035,6 +1035,11 @@
     // implementation, but this was not updated to match. Since then Android has
     // needed to maintain a stable libc++ ABI, and this can't be changed without
     // an ABI break.
+    // We also need this workaround for newlib since _NEWLIB_VERSION is not
+    // defined yet inside __config, so we can't set the
+    // _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE macro. Additionally, newlib is
+    // often used for space constrained environments, so it makes sense not to
+    // duplicate the ctype table.
     typedef uint16_t char_class_type;
 #else
     typedef ctype_base::mask        char_class_type;