Allow $ at the end of a bare word

Fixes #38

corpus/literals.txt

@@ -157,6 +157,18 @@ cat ${BAR} ${ABC=def} ${GHI:?jkl}
       (string (simple_expansion (variable_name)))
       (string (expansion (variable_name) (concatenation (word)))))))
 
+=============================
+Words ending with '$'
+=============================
+
+grep ^${var}$
+
+---
+
+(program (command
+  (command_name (word))
+  (concatenation (word) (expansion (variable_name)))))
+
 =============================
 Command substitutions
 =============================

corpus/statements.txt

@@ -252,6 +252,11 @@ Test commands with regexes
 [[ $CMD =~ (^|;)update_terminal_cwd($|;) ]]
 [[ ! " ${completions[*]} " =~ " $alias_cmd " ]]
 ! [[ "$a" =~ ^a|b\ *c|d$ ]]
+[[ "$1" =~ ^${var}${var}*=..* ]]
+[[ "$1" =~ ^\-${var}+ ]]
+[[ ${var1} == *${var2}* ]]
+[[ "$server" =~ [0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3} ]]
+[[ "$primary_wins" =~ ([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}) ]]
 
 ---
 
@@ -273,7 +278,27 @@ Test commands with regexes
     (test_command
       (binary_expression
         (string (simple_expansion (variable_name)))
-        (regex)))))
+        (regex))))
+  (test_command
+    (binary_expression
+      (string (simple_expansion (variable_name)))
+      (regex)))
+  (test_command
+    (binary_expression
+      (string (simple_expansion (variable_name)))
+      (regex)))
+  (test_command
+    (binary_expression
+      (expansion (variable_name))
+      (regex)))
+  (test_command
+    (binary_expression
+      (string (simple_expansion (variable_name)))
+      (regex)))
+  (test_command
+    (binary_expression
+      (string (simple_expansion (variable_name)))
+      (regex))))
 
 ===============================
 Subshells

grammar.js

@@ -404,6 +404,7 @@ module.exports = grammar({
           $._special_character,
         )
       ))),
+      optional(seq($._concat, '$'))
     )),
 
     _special_character: $ => token(prec(-1, choice('{', '}', '[', ']'))),

src/grammar.json

@@ -1615,6 +1615,27 @@
                 ]
               }
             }
+          },
+          {
+            "type": "CHOICE",
+            "members": [
+              {
+                "type": "SEQ",
+                "members": [
+                  {
+                    "type": "SYMBOL",
+                    "name": "_concat"
+                  },
+                  {
+                    "type": "STRING",
+                    "value": "$"
+                  }
+                ]
+              },
+              {
+                "type": "BLANK"
+              }
+            ]
           }
         ]
       }