This is a backreference, not an embedded NUL

Library/Homebrew/extend/ENV.rb

@@ -298,7 +298,7 @@ module HomebrewEnvExtension
 
     if compiler != :clang && Hardware.is_32_bit?
       # Can't mix "-march" for a 32-bit CPU  with "-arch x86_64"
-      replace_in_cflags(/-march=\S*/, "-Xarch_#{Hardware::CPU.arch_32_bit} \0")
+      replace_in_cflags(/-march=\S*/, "-Xarch_#{Hardware::CPU.arch_32_bit} \\0")
     end
   end