exp/regexp: add MustCompilePOSIX

R=r
CC=golang-dev
https://golang.org/cl/4962060
diff --git a/src/pkg/exp/regexp/exec_test.go b/src/pkg/exp/regexp/exec_test.go
index b6d9ece..e8eaff5 100644
--- a/src/pkg/exp/regexp/exec_test.go
+++ b/src/pkg/exp/regexp/exec_test.go
@@ -351,7 +351,7 @@
 	}
 }
 
-var notab = MustCompile(`[^\t]+`)
+var notab = MustCompilePOSIX(`[^\t]+`)
 
 func testFowler(t *testing.T, file string) {
 	f, err := os.Open(file)