improve conversion from header file name to once macro (#518)

Author: yudi-matsuzake

snippets/c-lang-common/once

@@ -2,7 +2,7 @@
 # name: #ifndef XXX; #define XXX; #endif
 # key: once
 # --
-#ifndef ${1:`(upcase (file-name-nondirectory (file-name-sans-extension (or (buffer-file-name) ""))))`_H}
+#ifndef ${1:`(upcase (replace-regexp-in-string "[^A-Za-z0-9_]" "_" (file-name-nondirectory (or (buffer-file-name)))))`}
 #define $1
 
 $0