Generalize parsing of expansion operators

corpus/expressions.txt

@@ -66,6 +66,27 @@ echo ${abc:
   (command (command_name (word)) (expansion (variable_name)))
   (command (command_name (word)) (expansion (variable_name))))
 
+===================================
+Variable expansions with operators
+===================================
+
+A="${B[0]# }"
+C="${D/#* -E /}"
+F="${G%% *}"
+
+---
+
+(program
+  (variable_assignment
+    (variable_name)
+    (string (expansion (subscript (variable_name) (word)))))
+  (variable_assignment
+    (variable_name)
+    (string (expansion (variable_name) (word) (word))))
+  (variable_assignment
+    (variable_name)
+    (string (expansion (variable_name) (word) (word)))))
+
 ===================================
 Variable expansions in strings
 ===================================

grammar.js

@@ -322,9 +322,7 @@ module.exports = grammar({
         seq(
           $.variable_name,
           '=',
-          optional(seq(
-            $._expression
-          ))
+          optional($._expression)
         ),
         seq(
           choice(
@@ -332,11 +330,9 @@ module.exports = grammar({
             $._simple_variable_name,
             $._special_variable_name
           ),
-          optional(seq(
-            choice(':', ':?', '=', ':-', '%', '/', '-'),
-            optional(seq(
-              $._expression
-            ))
+          repeat(choice(
+            $._expression,
+            ':', ':?', '=', ':-', '%', '/', '-', '#'
           ))
         ),
       ),