Allow variables to begin or end with underscores

corpus/env.txt

@@ -40,6 +40,23 @@ ENV X=42
       (unquoted_string)
       (unquoted_string))))
 
+================================================================================
+keys can begin or end with underscores
+================================================================================
+
+ENV _TEST="foo" TEST__=bar
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (env_instruction
+    (env_pair
+      name: (unquoted_string)
+      value: (double_quoted_string))
+    (env_pair
+      name: (unquoted_string)
+      value: (unquoted_string))))
+
 ================================================================================
 Multiple
 ================================================================================

corpus/vars.txt

@@ -86,6 +86,24 @@ ENV TEST="foo$BAR" \
         (expansion
           (variable))))))
 
+================================================================================
+Variables can begin or end with underscores
+================================================================================
+
+ENV TEST="$_foo${bar_}" 
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (env_instruction
+    (env_pair
+      (unquoted_string)
+      (double_quoted_string
+        (expansion
+          (variable))
+        (expansion
+          (variable))))))
+
 ================================================================================
 Empty Env
 ================================================================================

grammar.js

@@ -209,7 +209,7 @@ module.exports = grammar({
         )
       ),
 
-    variable: ($) => token.immediate(/[a-zA-Z][a-zA-Z0-9_]*/),
+    variable: ($) => token.immediate(/[a-zA-Z_][a-zA-Z0-9_]*/),
 
     env_pair: ($) =>
       seq(
@@ -228,7 +228,7 @@ module.exports = grammar({
       ),
 
     _env_key: ($) =>
-      alias(/[a-zA-Z]([a-zA-Z0-9_]*[a-zA-Z0-9])?/, $.unquoted_string),
+      alias(/[a-zA-Z_][a-zA-Z0-9_]*/, $.unquoted_string),
 
     expose_port: ($) => seq(/\d+/, optional(choice("/tcp", "/udp"))),
 

src/grammar.json

@@ -949,7 +949,7 @@
       "type": "IMMEDIATE_TOKEN",
       "content": {
         "type": "PATTERN",
-        "value": "[a-zA-Z][a-zA-Z0-9_]*"
+        "value": "[a-zA-Z_][a-zA-Z0-9_]*"
       }
     },
     "env_pair": {
@@ -1038,7 +1038,7 @@
       "type": "ALIAS",
       "content": {
         "type": "PATTERN",
-        "value": "[a-zA-Z]([a-zA-Z0-9_]*[a-zA-Z0-9])?"
+        "value": "[a-zA-Z_][a-zA-Z0-9_]*"
       },
       "named": true,
       "value": "unquoted_string"