diff --git a/crates/perry-runtime/src/regex.rs b/crates/perry-runtime/src/regex.rs
index da999c4c7a..e21f84b1f1 100644
--- a/crates/perry-runtime/src/regex.rs
+++ b/crates/perry-runtime/src/regex.rs
@@ -2040,6 +2040,51 @@ mod tests {
         }
     }
 
+    #[test]
+    fn annexb_legacy_decimal_escapes() {
+        // #5594: a `\<n>` with no matching capture group is an Annex B.1.4
+        // legacy octal escape, not a backreference — `\1` → `\x01`, never the
+        // bare `\1` the `regex`/`fancy-regex` crates reject.
+        assert_eq!(js_regex_to_rust(r"\1"), r"\x{01}");
+        assert_eq!(js_regex_to_rust(r"\b(\w+) \2\b"), r"\b(\w+) \x{02}\b");
+        // Multi-digit octal: `\12` = 0o12 = 0x0A, `\14` = 0o14 = 0x0C.
+        assert_eq!(js_regex_to_rust(r"[\12-\14]"), r"[\x{0A}-\x{0C}]");
+        // Inside a class a decimal escape is always octal, never a backref —
+        // even when that group exists.
+        assert_eq!(js_regex_to_rust(r"(a)[\1]"), r"(a)[\x{01}]");
+        // A real backward backreference is preserved for fancy-regex.
+        assert_eq!(js_regex_to_rust(r"(a)\1"), r"(a)\1");
+        // `\8` / `\9` are non-octal decimal escapes → literal digit.
+        assert_eq!(js_regex_to_rust(r"\8"), "8");
+        // `\0` is NUL; legacy `\012` = 0o12 = 0x0A.
+        assert_eq!(js_regex_to_rust(r"\0"), r"\x{00}");
+        assert_eq!(js_regex_to_rust(r"\012"), r"\x{0A}");
+
+        // The patterns that threw at construction must now compile and behave.
+        for pat in [r"\1", r"\b(\w+) \2\b", r"[\d][\12-\14]{1,}[^\d]"] {
+            let re = js_regexp_new(make_string(pat), make_string(""));
+            assert!(!re.is_null(), "pattern failed to construct: {pat}");
+        }
+    }
+
+    #[test]
+    fn annexb_invalid_control_escape_is_literal_backslash_c() {
+        // #5594: `\c` not followed by an ASCII control letter is the literal
+        // two-char sequence `\c`, not a control escape. The `regex`/`fancy-regex`
+        // crates reject a bare `\c`, so emit an escaped backslash + `c`.
+        assert_eq!(js_regex_to_rust(r"\cА"), r"\\cА"); // Cyrillic А (U+0410)
+        assert_eq!(js_regex_to_rust(r"\c "), r"\\c "); // space follows
+        assert_eq!(js_regex_to_rust(r"\c"), r"\\c"); // trailing
+        assert_eq!(js_regex_to_rust(r"[\c ]"), r"[\\c ]"); // inside a class
+                                                           // A valid control letter still lowers to its control byte (`\cA` = 0x01).
+        assert_eq!(js_regex_to_rust(r"\cA"), r"\x{01}");
+
+        for pat in [r"\cА", r"\c!", r"[\c ]"] {
+            let re = js_regexp_new(make_string(pat), make_string(""));
+            assert!(!re.is_null(), "pattern failed to construct: {pat}");
+        }
+    }
+
     #[test]
     fn surrogate_pairs_fold_to_astral_scalars() {
         // High escape + low class → contiguous astral range.
diff --git a/crates/perry-runtime/src/regex/grammar.rs b/crates/perry-runtime/src/regex/grammar.rs
index 1909c931ce..e011b26c75 100644
--- a/crates/perry-runtime/src/regex/grammar.rs
+++ b/crates/perry-runtime/src/regex/grammar.rs
@@ -13,6 +13,34 @@ fn parse_decimal_escape(chars: &[char], mut i: usize) -> (usize, usize) {
     (value, i - start)
 }
 
+/// Annex B.1.4: emit a `\<digits>` escape that is *not* a valid backreference as
+/// a `LegacyOctalEscapeSequence` (or a `NonOctalDecimalEscapeSequence` for a
+/// leading `8`/`9`). `start` indexes the first digit (just past the backslash).
+/// Returns the number of digit chars consumed.
+///
+/// `\1` with no group 1, or `\2` referencing a non-existent group, must compile
+/// as the literal byte rather than throwing — the `regex`/`fancy-regex` crates
+/// reject `\1` outright, so we lower it to `\x{HH}`.
+fn push_legacy_octal_escape(out: &mut String, chars: &[char], start: usize) -> usize {
+    let first = chars[start];
+    // `\8` / `\9` are not octal: they match the literal digit.
+    if first == '8' || first == '9' {
+        push_escaped_literal(out, first);
+        return 1;
+    }
+    // Up to three octal digits, but only two when the first is `4`–`7`
+    // (the value must stay ≤ 0o377 = 255).
+    let max = if matches!(first, '0'..='3') { 3 } else { 2 };
+    let mut value: u32 = 0;
+    let mut n = 0;
+    while n < max && start + n < chars.len() && matches!(chars[start + n], '0'..='7') {
+        value = value * 8 + (chars[start + n] as u32 - '0' as u32);
+        n += 1;
+    }
+    push_hex_escape(out, value as u8);
+    n
+}
+
 fn collect_capture_spans(chars: &[char]) -> Vec<CaptureSpan> {
     let mut spans = Vec::new();
     let mut stack: Vec<(usize, usize)> = Vec::new();
@@ -628,30 +656,56 @@ pub(super) fn js_regex_to_rust(pattern: &str) -> String {
                     result.push('/');
                     i += 2;
                 }
-                'c' if i + 2 < chars.len() => {
-                    if let Some(value) = control_escape_value(chars[i + 2]) {
+                'c' => {
+                    if let Some(value) = chars.get(i + 2).copied().and_then(control_escape_value) {
                         push_hex_escape(&mut result, value);
                         i += 3;
                     } else {
+                        // Annex B.1.4: a `\c` not followed by an ASCII control
+                        // letter (e.g. `\cА` with a Cyrillic letter, `\c$`, or a
+                        // trailing `\c`) is *not* a control escape — it is the
+                        // literal two-character sequence `\` `c`. Emit an escaped
+                        // backslash plus a literal `c` (the `regex`/`fancy-regex`
+                        // crates reject a bare `\c`); the following char, if any,
+                        // is processed normally so quantifiers/class members keep
+                        // their meaning. Works the same inside a `[...]` class.
+                        result.push('\\');
                         result.push('\\');
                         result.push('c');
                         i += 2;
                     }
                 }
-                '0' if i + 2 >= chars.len() || !chars[i + 2].is_ascii_digit() => {
-                    push_hex_escape(&mut result, 0);
-                    i += 2;
+                '0' => {
+                    // `\0` (NUL) and the legacy octal forms `\0DD` (Annex B.1.4)
+                    // — `push_legacy_octal_escape` consumes the octal run and
+                    // emits `\x{HH}`; a bare `\0` yields `\x00`.
+                    let consumed = push_legacy_octal_escape(&mut result, &chars, i + 1);
+                    i += 1 + consumed;
                 }
                 '1'..='9' => {
                     let (group, digits) = parse_decimal_escape(&chars, i + 1);
-                    if is_forward_backreference(&capture_spans, i, group) {
-                        i += 1 + digits;
-                    } else {
-                        result.push('\\');
-                        for ch in &chars[i + 1..i + 1 + digits] {
-                            result.push(*ch);
+                    // Inside a `[...]` class a decimal escape is never a
+                    // backreference — it is always a legacy octal/identity
+                    // escape (e.g. `[\12-\14]` is the range `\x0A`–`\x0C`).
+                    // Outside a class, `\<n>` is a backreference only when group
+                    // `n` actually exists; otherwise Annex B.1.4 reinterprets it.
+                    if !in_class && group <= capture_spans.len() {
+                        if is_forward_backreference(&capture_spans, i, group) {
+                            // A not-yet-closed group can't be matched by the
+                            // `regex`/`fancy-regex` engines; drop the reference.
+                            i += 1 + digits;
+                        } else {
+                            // A real backward backreference — keep it for
+                            // fancy-regex (the `regex` crate has no backrefs).
+                            result.push('\\');
+                            for ch in &chars[i + 1..i + 1 + digits] {
+                                result.push(*ch);
+                            }
+                            i += 1 + digits;
                         }
-                        i += 1 + digits;
+                    } else {
+                        let consumed = push_legacy_octal_escape(&mut result, &chars, i + 1);
+                        i += 1 + consumed;
                     }
                 }
                 'p' | 'P' if chars.get(i + 2) == Some(&'{') => {
diff --git a/test-files/test_issue_5594_regexp_annexb_escapes.ts b/test-files/test_issue_5594_regexp_annexb_escapes.ts
new file mode 100644
index 0000000000..80e15a14b3
--- /dev/null
+++ b/test-files/test_issue_5594_regexp_annexb_escapes.ts
@@ -0,0 +1,50 @@
+// #5594: Annex B legacy RegExp escapes — `\<n>` decimal escapes that aren't
+// backreferences become legacy octal/identity escapes (not the `\1` the regex
+// engines reject), and an invalid `\c<letter>` is the literal sequence `\c`.
+
+function check(name: string, value: boolean) {
+  if (!value) {
+    throw new Error(name);
+  }
+  console.log(name + ": ok");
+}
+
+// A `\<n>` with no matching capture group is a legacy octal escape, so the
+// pattern compiles and `\2` matches \x02 (absent here) instead of throwing.
+check("decimal-not-capturing", /\b(\w+) \2\b/.test("do you listen the the band") === false);
+
+// `.source` preserves the original pattern even though it lowered to \x01.
+check("leading-escape-source", /\1/.source === "\\1");
+check("leading-escape-a-source", /\a/.source === "\\a");
+check("trailing-escape-source", /a\1/.source === "a\\1");
+
+// Multi-digit legacy octal in a class range: `\12`=0o12=\n, `\14`=0o14=\x0C.
+const cls = /[\d][\12-\14]{1,}[^\d]/.exec("line1\n\n\n\n\nline2");
+check(
+  "decimal-class-range",
+  cls !== null && cls[0] === "1\n\n\n\n\nl" && cls.index === 4,
+);
+
+// `\8` / `\9` are non-octal decimal escapes → literal digit.
+check("non-octal-eight", /\8/.test("8") && !/\8/.test("a"));
+
+// A real backreference still works (fancy-regex path).
+const back = /(A)\1/.exec("AA");
+check("real-backref", back !== null && back[0] === "AA" && back[1] === "A");
+
+// Invalid `\c` (followed by a non-ASCII-letter) is the literal two chars `\c`.
+const cyrillic = String.fromCharCode(0x0410); // Cyrillic А
+const source = "\\c" + cyrillic;
+const re = new RegExp(source);
+check("invalid-control-no-wraparound", re.exec(String.fromCharCode(0x0410 % 32)) === null);
+check("invalid-control-not-c", re.exec(source.substring(1)) === null);
+check("invalid-control-matches-literal", re.exec(source) !== null);
+
+// Inside a character class, invalid `\c` contributes literal `\` and `c`.
+const classRe = new RegExp("[\\c" + cyrillic + "]");
+check("invalid-control-class-backslash", classRe.exec("\\") !== null);
+check("invalid-control-class-c", classRe.exec("c") !== null);
+
+// A valid control escape still lowers to its control byte.
+const ctrl = new RegExp("\\cA").exec(String.fromCharCode(1));
+check("valid-control-escape", ctrl !== null && ctrl[0] === String.fromCharCode(1));