1 print
"foo.txt".file_extension
.as(not null)
2 print
"foo.c".file_extension
.as(not null)
3 print
".txt".file_extension
.as(not null)
4 print
"my.file.txt".file_extension
.as(not null)
5 print
"my.".file_extension
.as(not null)
6 print
"file".file_extension
== null