diff --git a/grammar.js b/grammar.js
index 1907a3a3..60d0dd52 100644
--- a/grammar.js
+++ b/grammar.js
@@ -1560,7 +1560,7 @@ module.exports = grammar({
         seq('\\', choice(
           /[^xu]/,
           /u[0-9a-fA-F]{4}/,
-          /u\{[0-9a-fA-F]+\}/,
+          /u\{[0-9a-fA-F_]+\}/,
           /x[0-9a-fA-F]{2}/,
         )),
         /[^\\']/,
@@ -1573,7 +1573,7 @@ module.exports = grammar({
         choice(
           /[^xu]/,
           /u[0-9a-fA-F]{4}/,
-          /u\{[0-9a-fA-F]+\}/,
+          /u\{[0-9a-fA-F_]+\}/,
           /x[0-9a-fA-F]{2}/,
         ),
       )),
diff --git a/test/corpus/literals.txt b/test/corpus/literals.txt
index f6fa74de..be9f890b 100644
--- a/test/corpus/literals.txt
+++ b/test/corpus/literals.txt
@@ -81,6 +81,7 @@ b"foo\nbar";
 "/* foo bar */ foo bar";
 "foo\x42\x43bar";
 "foo \x42  \x43 bar";
+"\u{10__FFFF}";
 
 --------------------------------------------------------------------------------
 
@@ -123,7 +124,10 @@ b"foo\nbar";
       (escape_sequence)
       (string_content)
       (escape_sequence)
-      (string_content))))
+      (string_content)))
+  (expression_statement
+    (string_literal
+      (escape_sequence))))
 
 ================================================================================
 Raw string literals
@@ -198,6 +202,7 @@ b'x';
 '\t';
 '\xff';
 '\\';
+'\u{10__FFFF}';
 
 --------------------------------------------------------------------------------
 
@@ -214,6 +219,8 @@ b'x';
     (char_literal))
   (expression_statement
     (char_literal))
+  (expression_statement
+    (char_literal))
   (expression_statement
     (char_literal)))