[eggex] Reject an invalid pattern like / ! /

This was an error/typo in the grammar.

Also remove incomplete implementation of / ! %boundary /.  ERE doesn't
have any negated "zero-width assertions".

Addresses #1225.

oil_lang/grammar.pgen2

@@ -540,8 +540,9 @@ re_atom: (
     # %begin or %end
   | Expr_Symbol
   | class_literal
-    # !digit or ~ %boundary or ![a-f]
-  | '!' [Expr_Name | Expr_Symbol | class_literal]
+    # !digit or ![a-f].  Note ! %boundary could be \B in Python, but ERE
+    # doesn't have anything like that
+  | '!' (Expr_Name | class_literal)
     # Splice another expression
   | '@' Expr_Name
     # any %start %end are preferred

test/parse-errors.sh

@@ -698,6 +698,12 @@ json write (x) {
 regex_literals() {
   set +o errexit
 
+  _oil-parse-error 'var x = / ! /'
+  _oil-should-parse 'var x = / ![a-z] /'
+  _oil-should-parse 'var x = / !d /'
+
+  _oil-parse-error 'var x = / !! /'
+
   # missing space between rangfes
   _oil-parse-error 'var x = /[a-zA-Z]/'
   _oil-parse-error 'var x = /[a-z0-9]/'