fix(regular_expression): reject incomplete Unicode atom escapes (#28448)

Reject a consumed backslash without a valid Unicode AtomEscape,
including trailing escapes, and report the invalid escape at its source
span.

Validated with scoped Rust checks, full conformance, parser bindings,
the linter suite.

AI-assisted (Codex).

crates/oxc_linter/src/snapshots/eslint_no_invalid_regexp.snap

@@ -145,19 +145,19 @@ source: crates/oxc_linter/src/tester.rs
    ╰────
   help: Check for unbalanced brackets or invalid escape sequences
 
-  ⚠ eslint(no-invalid-regexp): Invalid regular expression: Could not parse the entire pattern
-   ╭─[no_invalid_regexp.tsx:1:15]
+  ⚠ eslint(no-invalid-regexp): Invalid regular expression: Invalid escape sequence
+   ╭─[no_invalid_regexp.tsx:1:13]
  1 │ new RegExp('\\a', 'u');
-   ·               ▲
+   ·             ──
    ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
-  ⚠ eslint(no-invalid-regexp): Invalid regular expression: Could not parse the entire pattern
-   ╭─[no_invalid_regexp.tsx:1:15]
+  ⚠ eslint(no-invalid-regexp): Invalid regular expression: Invalid escape sequence
+   ╭─[no_invalid_regexp.tsx:1:13]
  1 │ new RegExp('\\a', 'u');
-   ·               ▲
+   ·             ──
    ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
   ⚠ eslint(no-invalid-regexp): Invalid regular expression: Invalid braced quantifier
    ╭─[no_invalid_regexp.tsx:1:15]

crates/oxc_regular_expression/src/diagnostics.rs

@@ -75,6 +75,13 @@ pub fn invalid_extended_atom_escape(span: Span) -> OxcDiagnostic {
         .with_help("Use a valid escape sequence or remove the backslash")
 }
 
+#[cold]
+pub fn invalid_atom_escape(span: Span) -> OxcDiagnostic {
+    OxcDiagnostic::error(format!("{PREFIX} Invalid escape sequence"))
+        .with_label(span)
+        .with_help("Use a valid escape sequence or escape the backslash with `\\\\`")
+}
+
 #[cold]
 pub fn invalid_braced_quantifier(span: Span) -> OxcDiagnostic {
     OxcDiagnostic::error(format!("{PREFIX} Invalid braced quantifier"))

crates/oxc_regular_expression/src/parser/mod.rs

@@ -12,6 +12,40 @@ mod test {
 
     use crate::{ConstructorParser, LiteralParser, Options, ast::Term};
 
+    #[test]
+    fn trailing_escape() {
+        let allocator = Allocator::default();
+        for flags in ["", "u", "v"] {
+            for pattern in [r"\", r"a\", r"a|\"] {
+                assert!(
+                    LiteralParser::new(&allocator, pattern, Some(flags), Options::default())
+                        .parse()
+                        .is_err(),
+                    "/{pattern}/{flags}"
+                );
+            }
+            let flags_source = format!("\"{flags}\"");
+            for source in [r#""\\""#, r#""a\\""#, r"`a\\`"] {
+                assert!(
+                    ConstructorParser::new(
+                        &allocator,
+                        source,
+                        Some(&flags_source),
+                        Options::default()
+                    )
+                    .parse()
+                    .is_err(),
+                    "new RegExp({source}, {flags_source})"
+                );
+            }
+            assert!(
+                LiteralParser::new(&allocator, r"\\", Some(flags), Options::default())
+                    .parse()
+                    .is_ok()
+            );
+        }
+    }
+
     #[test]
     fn escaped_group_names() {
         let allocator = Allocator::default();

crates/oxc_regular_expression/src/parser/pattern_parser/pattern_parser_impl.rs

@@ -359,10 +359,12 @@ impl<'a> PatternParser<'a> {
         }
 
         // \ AtomEscape[?UnicodeMode, ?NamedCaptureGroups]
-        if self.reader.eat('\\')
-            && let Some(atom_escape) = self.parse_atom_escape(span_start)?
-        {
-            return Ok(Some(atom_escape));
+        if self.reader.eat('\\') {
+            return self.parse_atom_escape(span_start)?.map(Some).ok_or_else(|| {
+                diagnostics::invalid_atom_escape(
+                    self.span_factory.create(span_start, self.reader.offset()),
+                )
+            });
         }
 
         // CharacterClass[?UnicodeMode, ?UnicodeSetsMode]

crates/oxc_regular_expression/tests/snapshots/diagnostics__test.snap

@@ -283,12 +283,12 @@ expression: snapshot
 
 # invalid_extended_atom_escape
 
-  × Invalid regular expression: Could not parse the entire pattern
-   ╭─[/\c0/u:1:3]
+  × Invalid regular expression: Invalid escape sequence
+   ╭─[/\c0/u:1:2]
  1 │ /\c0/u
-   ·   ▲
+   ·  ─
    ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
 # duplicated_modifiers
 

tasks/coverage/snapshots/parser_misc.snap

@@ -5144,12 +5144,12 @@ Negative Passed: 265/265 (100.00%)
    ╰────
   help: This character or escape sequence is not valid inside a character class
 
-  × Invalid regular expression: Could not parse the entire pattern
-   ╭─[misc/fail/regexp-unicode-leading-zero-reference.js:1:5]
+  × Invalid regular expression: Invalid escape sequence
+   ╭─[misc/fail/regexp-unicode-leading-zero-reference.js:1:4]
  1 │ /()\01/u;
-   ·     ▲
+   ·    ─
    ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
   × Invalid rest element target in destructuring assignment
    ╭─[misc/fail/rest-assignment-target.js:1:6]

tasks/coverage/snapshots/parser_test262.snap

@@ -24615,13 +24615,13 @@ Negative Passed: 4653/4653 (100.00%)
     ╰────
   help: Add the missing closing delimiter
 
-  × Invalid regular expression: Unterminated capturing group
-    ╭─[test262/test/language/literals/regexp/named-groups/invalid-identity-escape-in-capture-u.js:16:2]
+  × Invalid regular expression: Invalid escape sequence
+    ╭─[test262/test/language/literals/regexp/named-groups/invalid-identity-escape-in-capture-u.js:16:7]
  15 │ 
  16 │ /(?<a>\a)/u;
-    ·  ──────
+    ·       ─
     ╰────
-  help: Add the missing closing delimiter
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
   × Invalid regular expression: Unterminated capturing group name
     ╭─[test262/test/language/literals/regexp/named-groups/invalid-incomplete-groupname-2-u.js:16:11]
@@ -24991,13 +24991,13 @@ Negative Passed: 4653/4653 (100.00%)
  34 │ ╰─▶ */
     ╰────
 
-  × Invalid regular expression: Could not parse the entire pattern
-    ╭─[test262/test/language/literals/regexp/u-invalid-class-escape.js:24:3]
+  × Invalid regular expression: Invalid escape sequence
+    ╭─[test262/test/language/literals/regexp/u-invalid-class-escape.js:24:2]
  23 │ 
  24 │ /\c0/u;
-    ·   ▲
+    ·  ─
     ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
   × Invalid regular expression: Could not parse the entire pattern
     ╭─[test262/test/language/literals/regexp/u-invalid-extended-pattern-char.js:21:2]
@@ -25007,13 +25007,13 @@ Negative Passed: 4653/4653 (100.00%)
     ╰────
   help: Check for unbalanced brackets or invalid escape sequences
 
-  × Invalid regular expression: Could not parse the entire pattern
-    ╭─[test262/test/language/literals/regexp/u-invalid-identity-escape.js:23:3]
+  × Invalid regular expression: Invalid escape sequence
+    ╭─[test262/test/language/literals/regexp/u-invalid-identity-escape.js:23:2]
  22 │ 
  23 │ /\M/u;
-    ·   ▲
+    ·  ─
     ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
   × Invalid regular expression: Invalid indexed reference
     ╭─[test262/test/language/literals/regexp/u-invalid-legacy-octal-escape.js:26:2]

tasks/coverage/snapshots/parser_typescript.snap

@@ -12389,14 +12389,14 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
     ╰────
   help: Duplicate names must be in different alternatives
 
-  × Invalid regular expression: Could not parse the entire pattern
-   ╭─[typescript/tsc/testdata/tests/cases/compiler/regularExpressionAnnexB.ts:7:5]
+  × Invalid regular expression: Invalid escape sequence
+   ╭─[typescript/tsc/testdata/tests/cases/compiler/regularExpressionAnnexB.ts:7:4]
  6 │   // Compare to
  7 │   /\q\u\i\c\k\_\f\o\x\-\j\u\m\p\s/u,
-   ·     ▲
+   ·    ─
  8 │   /[\q\u\i\c\k\_\f\o\x\-\j\u\m\p\s]/u,
    ╰────
-  help: Check for unbalanced brackets or invalid escape sequences
+  help: Use a valid escape sequence or escape the backslash with `\\`
 
   × Invalid regular expression: Invalid class atom
    ╭─[typescript/tsc/testdata/tests/cases/compiler/regularExpressionAnnexB.ts:8:5]