diff --git a/src/blackmisc/aviation/altitude.cpp b/src/blackmisc/aviation/altitude.cpp
index 6e730fd62..adbd5877c 100644
--- a/src/blackmisc/aviation/altitude.cpp
+++ b/src/blackmisc/aviation/altitude.cpp
@@ -385,7 +385,7 @@ namespace BlackMisc
 
         const QRegularExpression &CAltitude::fpAltitudeRegExp()
         {
-            thread_local const QRegularExpression re("((FL|F)\\d{2,3})|(S\\d{2,4})|(A\\d{2,3})|(M\\d{2,4})|(\\d{3,5}(ft|m))");
+            thread_local const QRegularExpression re("([Ff][Ll]?\\d{2,3})|([Ss]\\d{2,4})|([Aa]\\d{2,3})|([Mm]\\d{2,4})|(\\d{3,5}(ft|m))");
             return re;
         }