Allow empty strings

corpus/declarations.txt

@@ -37,3 +37,21 @@ a {
     (block
       (declaration (property_name) (color_value))
       (declaration (property_name) (color_value)))))
+
+============================
+Strings
+============================
+
+a {
+  b: '';
+  c: '\'hi\'';
+}
+
+---
+
+(stylesheet
+  (rule_set
+    (selectors (tag_name))
+    (block
+      (declaration (property_name) (string_value))
+      (declaration (property_name) (string_value)))))

grammar.js

@@ -191,8 +191,8 @@ module.exports = grammar({
     color_value: $ => /#[0-9a-fA-F]{3,8}/,
 
     string_value: $ => token(choice(
-      seq("'", /([^'\n]|\\(.|\n))+/, "'"),
+      seq("'", /([^'\n]|\\(.|\n))*/, "'"),
-      seq('"', /([^"\n]|\\(.|\n))+/, '"')
+      seq('"', /([^"\n]|\\(.|\n))*/, '"')
     )),
 
     integer_value: $ => seq(

src/grammar.json

@@ -810,7 +810,7 @@
               },
               {
                 "type": "PATTERN",
-                "value": "([^']|\\\\.)+"
+                "value": "([^'\\n]|\\\\(.|\\n))*"
               },
               {
                 "type": "STRING",
@@ -827,7 +827,7 @@
               },
               {
                 "type": "PATTERN",
-                "value": "([^\"]|\\\\.)+"
+                "value": "([^\"\\n]|\\\\(.|\\n))*"
               },
               {
                 "type": "STRING",