diff --git a/R/guess.r b/R/guess.r
index 1224faaa..f264633c 100644
--- a/R/guess.r
+++ b/R/guess.r
@@ -383,14 +383,14 @@ guess_formats <- function(x, orders, locale = Sys.getlocale("LC_TIME"),
                r = sprintf("((%s\\D*%s)|%s)",
                  num[["I"]], alpha[["p"]], num[["H"]]))
       
-      num[nms] <- sub("<M", "<M_T", 
-                      sub("<S", "<S_T", 
-                          sub("<OS", "<OS_T", num[nms])))
+      num[nms] <- sub("<M", "<M_T", fixed = TRUE, 
+                      sub("<S", "<S_T", fixed = TRUE, 
+                          sub("<OS", "<OS_T", num[nms], fixed = TRUE)))
     }
 
 
     ## don't let special nms be confused with standard formats
-    num[nms] <- gsub("(<[IMSpHS]|<OS)", "\\1_s", num[nms]) 
+    num[nms] <- gsub("(<[IMSpHS]|<OS)", "\\1_s", num[nms], perl = TRUE)
     
 
     ## The difference between flex regexp and exact is that flex should follow