swc-project · Felix-Ayush (Ayush7614) · Jul 31, 2026 · Jul 31, 2026 · Jul 31, 2026
@@ -0,0 +1,6 @@
+---
+swc_core: patch
+swc_ecma_parser: patch
+---
+
+fix(es/parser): reject unicode escapes in RegExp flags
@@ -213,6 +213,8 @@ pub enum SyntaxError {
 
     DuplicatedRegExpFlags(char),
     UnknownRegExpFlags,
+    /// IdentifierPart in RegExp flags must not contain a UnicodeEscapeSequence.
+    UnicodeEscapeInRegExpFlags,
 
     TS1003,
     TS1005,
@@ -591,6 +593,9 @@ impl SyntaxError {
                 format!("Duplicated regular expression flag '{flag}'.").into()
             }
             SyntaxError::UnknownRegExpFlags => "Unknown regular expression flags.".into(),
+            SyntaxError::UnicodeEscapeInRegExpFlags => {
+                "Regular expression flags cannot contain unicode escapes.".into()
+            }
 
             SyntaxError::TS1003 => "Expected an identifier".into(),
             SyntaxError::TS1005 => "Expected a semicolon".into(),

@@ -1870,16 +1870,21 @@ impl<'a> Lexer<'a> {
         self.bump(1); // '/'
 
         // Spec says "It is a Syntax Error if IdentifierPart contains a Unicode escape
-        // sequence." TODO: check for escape
-
-        // Need to use `read_word` because '\uXXXX' sequences are allowed
-        // here (don't ask).
-        // let flags_start = self.cur_pos();
+        // sequence."
+        // Need to use `read_word` because '\uXXXX' sequences are accepted by the
+        // scanner — we still reject them as an early error below.
+        // Use `cur_as_char` so non-ASCII IdentifierPart flags (e.g. `/a/π\u0067`)
+        // are scanned; `cur()` only yields the first UTF-8 byte and would miss them.
         let flags = {
-            match self.cur() {
-                Some(c) if c.is_ident_start() => self
-                    .read_word_as_str_with()
-                    .map(|(s, _)| Some(self.atom(s))),
+            match self.cur_as_char() {
+                Some(c) if c == '\\' || c.is_ident_part() => {
+                    let (s, has_escape) = self.read_word_as_str_with()?;
+                    if has_escape {
+                        let span = self.span(start);
+                        self.emit_error_span(span, SyntaxError::UnicodeEscapeInRegExpFlags);
+                    }
+                    Ok(Some(self.atom(s)))
+                }
                 _ => Ok(None),
             }
         }?;

@@ -0,0 +1,3 @@
+/pattern/g\u0069m;
+/pattern/\u0067im;
+/pattern/gi\u006d;
@@ -0,0 +1,19 @@
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/escaped-among-ascii/input.js:1:1]
+ 1 | /pattern/g\u0069m;
+   : ^^^^^^^^^^^^^^^^^
+ 2 | /pattern/\u0067im;
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/escaped-among-ascii/input.js:2:1]
+ 1 | /pattern/g\u0069m;
+ 2 | /pattern/\u0067im;
+   : ^^^^^^^^^^^^^^^^^
+ 3 | /pattern/gi\u006d;
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/escaped-among-ascii/input.js:3:1]
+ 2 | /pattern/\u0067im;
+ 3 | /pattern/gi\u006d;
+   : ^^^^^^^^^^^^^^^^^
+   `----
@@ -0,0 +1,4 @@
+/ok/gimuy;
+/bad/g\u0069m;
+/also/\u0067;
+/leading/\u{79};
@@ -0,0 +1,20 @@
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/mixed-valid-invalid/input.js:2:1]
+ 1 | /ok/gimuy;
+ 2 | /bad/g\u0069m;
+   : ^^^^^^^^^^^^^
+ 3 | /also/\u0067;
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/mixed-valid-invalid/input.js:3:1]
+ 2 | /bad/g\u0069m;
+ 3 | /also/\u0067;
+   : ^^^^^^^^^^^^
+ 4 | /leading/\u{79};
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/mixed-valid-invalid/input.js:4:1]
+ 3 | /also/\u0067;
+ 4 | /leading/\u{79};
+   : ^^^^^^^^^^^^^^^
+   `----
@@ -0,0 +1 @@
+/a/π\u0067;
@@ -0,0 +1,10 @@
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/non-ascii-start/input.js:1:1]
+ 1 | /a/π\u0067;
+   : ^^^^^^^^^^
+   `----
+  x Unknown regular expression flags.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/non-ascii-start/input.js:1:1]
+ 1 | /a/π\u0067;
+   : ^^^^^^^^^^
+   `----
@@ -0,0 +1,2 @@
+/x/\u0067;
+/x/\u{0069};
@@ -0,0 +1,12 @@
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/only-escaped-flag/input.js:1:1]
+ 1 | /x/\u0067;
+   : ^^^^^^^^^
+ 2 | /x/\u{0069};
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/only-escaped-flag/input.js:2:1]
+ 1 | /x/\u0067;
+ 2 | /x/\u{0069};
+   : ^^^^^^^^^^^
+   `----
@@ -0,0 +1,3 @@
+/a/\u{69};
+/foo/g\u{0069};
+/\w+/\u{006d}\u{0073};
@@ -0,0 +1,19 @@
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/u-brace-escape/input.js:1:1]
+ 1 | /a/\u{69};
+   : ^^^^^^^^^
+ 2 | /foo/g\u{0069};
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/u-brace-escape/input.js:2:1]
+ 1 | /a/\u{69};
+ 2 | /foo/g\u{0069};
+   : ^^^^^^^^^^^^^^
+ 3 | /\w+/\u{006d}\u{0073};
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/u-brace-escape/input.js:3:1]
+ 2 | /foo/g\u{0069};
+ 3 | /\w+/\u{006d}\u{0073};
+   : ^^^^^^^^^^^^^^^^^^^^^
+   `----
@@ -0,0 +1,2 @@
+/a/g\u0069;
+/\w+/m\u0073;
@@ -0,0 +1,12 @@
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/u-escape/input.js:1:1]
+ 1 | /a/g\u0069;
+   : ^^^^^^^^^^
+ 2 | /\w+/m\u0073;
+   `----
+  x Regular expression flags cannot contain unicode escapes.
+   ,-[$DIR/tests/errors/regexp-flags-unicode-escape/u-escape/input.js:2:1]
+ 1 | /a/g\u0069;
+ 2 | /\w+/m\u0073;
+   : ^^^^^^^^^^^^
+   `----
@@ -3,8 +3,3 @@
  1 | var x = /[a-z]/\ux
    :                  ^
    `----
-  x Expected a semicolon
-   ,-[$DIR/tests/test262-parser/fail/6a96389a0cce57e9.js:1:1]
- 1 | var x = /[a-z]/\ux
-   :                ^^
-   `----
@@ -3,8 +3,3 @@
  1 | var x = /[a-z]/\\ux
    :                ^
    `----
-  x Expected a semicolon
-   ,-[$DIR/tests/test262-parser/fail/94535dc25ef762ee.js:1:1]
- 1 | var x = /[a-z]/\\ux
-   :                ^
-   `----
@@ -3,8 +3,3 @@
  1 | var x = /[P QR]/\\u0067
    :                 ^
    `----
-  x Expected a semicolon
-   ,-[$DIR/tests/test262-parser/fail/e7087ec92f0f3c44.js:1:1]
- 1 | var x = /[P QR]/\\u0067
-   :                 ^
-   `----