diff --git a/libcxx/test/libcxx/selftest/additional_compile_flags/substitutes-in-compile-flags.sh.cpp b/libcxx/test/libcxx/selftest/additional_compile_flags/substitutes-in-compile-flags.sh.cpp
--- a/libcxx/test/libcxx/selftest/additional_compile_flags/substitutes-in-compile-flags.sh.cpp
+++ b/libcxx/test/libcxx/selftest/additional_compile_flags/substitutes-in-compile-flags.sh.cpp
@@ -9,10 +9,11 @@
 // This test greps for %t, which is expanded to a path with backslashes. When
 // that is passed to grep, those backslashes would have to be escaped, which we
 // don't do right now.
-// UNSUPPORTED: windows
 
 // Make sure that substitutions are performed inside additional compiler flags.
 
 // ADDITIONAL_COMPILE_FLAGS: -I %t.1
 // ADDITIONAL_COMPILE_FLAGS: -isystem %t.2 , -isysroot %t.3
-// RUN: echo "%{compile_flags}" | grep -e '-I %t.1 -isystem %t.2 -isysroot %t.3'
+// RUN: echo "-I %t.1 -isystem %t.2 -isysroot %t.3" | sed "s/\\\/\\\\\\\/g" > %t.grep
+// RUN: echo "%{compile_flags}" | grep -e -f %t.grep
+// RUN: rm %t.grep