Hi Muxuan,
1. You can pass a regular expression to test.text, so in your case you would do something like getTextInput("myTextInput").test.text( /^[sS]\d+/ )
2. This could be due to your TextInput element printing almost immediately after the key press on F, resulting in that character printing in the text input box. Maybe try starting and waiting for a 250ms Timer after the Key element’s wait command and before the newTextInput command to see if things improve
Jeremy