diff --git a/libcxx/include/__locale b/libcxx/include/__locale
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -496,7 +496,13 @@
     static const mask punct  = 1<<7;
     static const mask xdigit = 1<<8;
     static const mask blank  = 1<<9;
+#if defined(__BIONIC__)
+    // Historically this was a part of regex_traits rather than ctype_base. The
+    // historical value of the constant is preserved for ABI compatibility.
+    static const mask __regex_word = 0x8000;
+#else
     static const mask __regex_word = 1<<10;
+#endif // defined(__BIONIC__)
 #endif
     static const mask alnum  = alpha | digit;
     static const mask graph  = alnum | punct;
diff --git a/libcxx/include/regex b/libcxx/include/regex
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -1000,7 +1000,19 @@
     typedef _CharT                  char_type;
     typedef basic_string<char_type> string_type;
     typedef locale                  locale_type;
+#ifdef __BIONIC__
+    // 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
+    // to make room for __regex_word (then a part of this class rather than
+    // ctype_base). Bionic has since moved to the builtin ctype_base
+    // 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.
+    typedef uint16_t char_class_type;
+#else
     typedef ctype_base::mask        char_class_type;
+#endif
 
     static const char_class_type __regex_word = ctype_base::__regex_word;
 private: