Add simple atoms

grammar.js

@@ -2,8 +2,39 @@ module.exports = grammar({
   name: "tiger",
 
   rules: {
-    // TODO: add the actual grammar rules
-    source_file: $ => 'hello'
+    source_file: ($) => choice(
+      $._expr,
+    ),
+
+    _expr: ($) => choice(
+      "nil",
+      $.integer_literal,
+      $.string_literal,
+    ),
+
+    integer_literal: (_) => /[0-9]+/,
+
+    string_literal: ($) => seq(
+      '"',
+      repeat(choice($.escape_sequence, /[^"\\]+/)),
+      '"',
+    ),
+
+    escape_sequence: (_) => token.immediate(
+      seq(
+        "\\",
+        choice(
+          // Special escapes
+          choice("a", "b", "f", "n", "r", "t", "v"),
+          // Octal
+          /[0-3][0-7]{2}/,
+          // Hexadecimal
+          seq("x", /[0-9a-fA-F]{2}/),
+          // Escaped characters
+          choice("\\", '"'),
+        )
+      )
+    ),
   }
 });