Encoding name can contain underscores.

pep-0263.txt

@@ -61,7 +61,7 @@ Defining the Encoding
           # vim: set fileencoding=<encoding name> :
 
     More precisely, the first or second line must match the regular
-    expression "^[ \t\v]*#.*?coding[:=][ \t]*([-.a-zA-Z0-9]+)".
+    expression "^[ \t\v]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)".
     The first group of this
     expression is then interpreted as encoding name. If the encoding
     is unknown to Python, an error is raised during compilation. There