diff --git a/lib/js/conformance.sh b/lib/js/conformance.sh
index c6f91502..b181e9ad 100755
--- a/lib/js/conformance.sh
+++ b/lib/js/conformance.sh
@@ -49,6 +49,8 @@ trap "rm -f $TMPFILE" EXIT
   echo '(load "lib/js/transpile.sx")'
   echo '(epoch 5)'
   echo '(load "lib/js/runtime.sx")'
+  echo '(epoch 6)'
+  echo '(load "lib/js/regex.sx")'
 
   epoch=100
   for f in "${FIXTURES[@]}"; do
diff --git a/lib/js/regex.sx b/lib/js/regex.sx
new file mode 100644
index 00000000..c56957ca
--- /dev/null
+++ b/lib/js/regex.sx
@@ -0,0 +1,943 @@
+;; lib/js/regex.sx — pure-SX recursive backtracking regex engine
+;;
+;; Installed via (js-regex-platform-override! ...) at load time.
+;; Covers: character classes (\d\w\s . [abc] [^abc] [a-z]),
+;;   anchors (^ $ \b \B), quantifiers (* + ? {n,m} lazy variants),
+;;   groups (capturing + non-capturing), alternation (a|b),
+;;   flags: i (case-insensitive), g (global), m (multiline).
+;;
+;; Architecture:
+;;   1. rx-parse-pattern  — pattern string → compiled node list
+;;   2. rx-match-nodes    — recursive backtracker
+;;   3. rx-exec / rx-test — public interface
+;;   4. Install as {:test rx-test :exec rx-exec}
+
+;; ── Utilities ─────────────────────────────────────────────────────
+
+(define
+  rx-char-at
+  (fn (s i) (if (and (>= i 0) (< i (len s))) (char-at s i) "")))
+
+(define
+  rx-digit?
+  (fn
+    (c)
+    (and (not (= c "")) (>= (char-code c) 48) (<= (char-code c) 57))))
+
+(define
+  rx-word?
+  (fn
+    (c)
+    (and
+      (not (= c ""))
+      (or
+        (and (>= (char-code c) 65) (<= (char-code c) 90))
+        (and (>= (char-code c) 97) (<= (char-code c) 122))
+        (and (>= (char-code c) 48) (<= (char-code c) 57))
+        (= c "_")))))
+
+(define
+  rx-space?
+  (fn
+    (c)
+    (or (= c " ") (= c "\t") (= c "\n") (= c "\r") (= c "\\f") (= c ""))))
+
+(define rx-newline? (fn (c) (or (= c "\n") (= c "\r"))))
+
+(define
+  rx-downcase-char
+  (fn
+    (c)
+    (let
+      ((cc (char-code c)))
+      (if (and (>= cc 65) (<= cc 90)) (char-from-code (+ cc 32)) c))))
+
+(define
+  rx-char-eq?
+  (fn
+    (a b ci?)
+    (if ci? (= (rx-downcase-char a) (rx-downcase-char b)) (= a b))))
+
+(define
+  rx-parse-int
+  (fn
+    (pat i acc)
+    (let
+      ((c (rx-char-at pat i)))
+      (if
+        (rx-digit? c)
+        (rx-parse-int pat (+ i 1) (+ (* acc 10) (- (char-code c) 48)))
+        (list acc i)))))
+
+(define
+  rx-hex-digit-val
+  (fn
+    (c)
+    (cond
+      ((and (>= (char-code c) 48) (<= (char-code c) 57))
+        (- (char-code c) 48))
+      ((and (>= (char-code c) 65) (<= (char-code c) 70))
+        (+ 10 (- (char-code c) 65)))
+      ((and (>= (char-code c) 97) (<= (char-code c) 102))
+        (+ 10 (- (char-code c) 97)))
+      (else -1))))
+
+(define
+  rx-parse-hex-n
+  (fn
+    (pat i n acc)
+    (if
+      (= n 0)
+      (list (char-from-code acc) i)
+      (let
+        ((v (rx-hex-digit-val (rx-char-at pat i))))
+        (if
+          (< v 0)
+          (list (char-from-code acc) i)
+          (rx-parse-hex-n pat (+ i 1) (- n 1) (+ (* acc 16) v)))))))
+
+;; ── Pattern compiler ──────────────────────────────────────────────
+
+;; Node types (stored in dicts with "__t__" key):
+;;   literal  : {:__t__ "literal" :__c__ char}
+;;   any      : {:__t__ "any"}
+;;   class-d  : {:__t__ "class-d"  :__neg__ bool}
+;;   class-w  : {:__t__ "class-w"  :__neg__ bool}
+;;   class-s  : {:__t__ "class-s"  :__neg__ bool}
+;;   char-class: {:__t__ "char-class" :__neg__ bool :__items__ list}
+;;   anchor-start / anchor-end / anchor-word / anchor-nonword
+;;   quant    : {:__t__ "quant" :__node__ n :__min__ m :__max__ mx :__lazy__ bool}
+;;   group    : {:__t__ "group" :__idx__ i :__nodes__ list}
+;;   ncgroup  : {:__t__ "ncgroup" :__nodes__ list}
+;;   alt      : {:__t__ "alt" :__branches__ list-of-node-lists}
+
+;; parse one escape after `\`, returns (node new-i)
+(define
+  rx-parse-escape
+  (fn
+    (pat i)
+    (let
+      ((c (rx-char-at pat i)))
+      (cond
+        ((= c "d") (list (dict "__t__" "class-d" "__neg__" false) (+ i 1)))
+        ((= c "D") (list (dict "__t__" "class-d" "__neg__" true) (+ i 1)))
+        ((= c "w") (list (dict "__t__" "class-w" "__neg__" false) (+ i 1)))
+        ((= c "W") (list (dict "__t__" "class-w" "__neg__" true) (+ i 1)))
+        ((= c "s") (list (dict "__t__" "class-s" "__neg__" false) (+ i 1)))
+        ((= c "S") (list (dict "__t__" "class-s" "__neg__" true) (+ i 1)))
+        ((= c "b") (list (dict "__t__" "anchor-word") (+ i 1)))
+        ((= c "B") (list (dict "__t__" "anchor-nonword") (+ i 1)))
+        ((= c "n") (list (dict "__t__" "literal" "__c__" "\n") (+ i 1)))
+        ((= c "r") (list (dict "__t__" "literal" "__c__" "\r") (+ i 1)))
+        ((= c "t") (list (dict "__t__" "literal" "__c__" "\t") (+ i 1)))
+        ((= c "f") (list (dict "__t__" "literal" "__c__" "\\f") (+ i 1)))
+        ((= c "v") (list (dict "__t__" "literal" "__c__" "") (+ i 1)))
+        ((= c "u")
+          (let
+            ((res (rx-parse-hex-n pat (+ i 1) 4 0)))
+            (list (dict "__t__" "literal" "__c__" (nth res 0)) (nth res 1))))
+        ((= c "x")
+          (let
+            ((res (rx-parse-hex-n pat (+ i 1) 2 0)))
+            (list (dict "__t__" "literal" "__c__" (nth res 0)) (nth res 1))))
+        (else (list (dict "__t__" "literal" "__c__" c) (+ i 1)))))))
+
+;; parse a char-class item inside [...], returns (item new-i)
+(define
+  rx-parse-class-item
+  (fn
+    (pat i)
+    (let
+      ((c (rx-char-at pat i)))
+      (cond
+        ((= c "\\")
+          (let
+            ((esc (rx-parse-escape pat (+ i 1))))
+            (let
+              ((node (nth esc 0)) (ni (nth esc 1)))
+              (let
+                ((t (get node "__t__")))
+                (cond
+                  ((= t "class-d")
+                    (list
+                      (dict "kind" "class-d" "neg" (get node "__neg__"))
+                      ni))
+                  ((= t "class-w")
+                    (list
+                      (dict "kind" "class-w" "neg" (get node "__neg__"))
+                      ni))
+                  ((= t "class-s")
+                    (list
+                      (dict "kind" "class-s" "neg" (get node "__neg__"))
+                      ni))
+                  (else
+                    (let
+                      ((lc (get node "__c__")))
+                      (if
+                        (and
+                          (= (rx-char-at pat ni) "-")
+                          (not (= (rx-char-at pat (+ ni 1)) "]")))
+                        (let
+                          ((hi-c (rx-char-at pat (+ ni 1))))
+                          (list
+                            (dict "kind" "range" "lo" lc "hi" hi-c)
+                            (+ ni 2)))
+                        (list (dict "kind" "lit" "c" lc) ni)))))))))
+        (else
+          (if
+            (and
+              (not (= c ""))
+              (= (rx-char-at pat (+ i 1)) "-")
+              (not (= (rx-char-at pat (+ i 2)) "]"))
+              (not (= (rx-char-at pat (+ i 2)) "")))
+            (let
+              ((hi-c (rx-char-at pat (+ i 2))))
+              (list (dict "kind" "range" "lo" c "hi" hi-c) (+ i 3)))
+            (list (dict "kind" "lit" "c" c) (+ i 1))))))))
+
+(define
+  rx-parse-class-items
+  (fn
+    (pat i items)
+    (let
+      ((c (rx-char-at pat i)))
+      (if
+        (or (= c "]") (= c ""))
+        (list items i)
+        (let
+          ((res (rx-parse-class-item pat i)))
+          (begin
+            (append! items (nth res 0))
+            (rx-parse-class-items pat (nth res 1) items)))))))
+
+;; parse a sequence until stop-ch or EOF; returns (nodes new-i groups-count)
+(define
+  rx-parse-seq
+  (fn
+    (pat i stop-ch ds)
+    (let
+      ((c (rx-char-at pat i)))
+      (cond
+        ((= c "") (list (get ds "nodes") i (get ds "groups")))
+        ((= c stop-ch) (list (get ds "nodes") i (get ds "groups")))
+        ((= c "|") (rx-parse-alt-rest pat i ds))
+        (else
+          (let
+            ((res (rx-parse-atom pat i ds)))
+            (let
+              ((node (nth res 0)) (ni (nth res 1)) (ds2 (nth res 2)))
+              (let
+                ((qres (rx-parse-quant pat ni node)))
+                (begin
+                  (append! (get ds2 "nodes") (nth qres 0))
+                  (rx-parse-seq pat (nth qres 1) stop-ch ds2))))))))))
+
+;; when we hit | inside a sequence, collect all alternatives
+(define
+  rx-parse-alt-rest
+  (fn
+    (pat i ds)
+    (let
+      ((left-branch (get ds "nodes")) (branches (list)))
+      (begin
+        (append! branches left-branch)
+        (rx-parse-alt-branches pat i (get ds "groups") branches)))))
+
+(define
+  rx-parse-alt-branches
+  (fn
+    (pat i n-groups branches)
+    (let
+      ((new-nodes (list)) (ds2 (dict "groups" n-groups "nodes" new-nodes)))
+      (let
+        ((res (rx-parse-seq pat (+ i 1) "|" ds2)))
+        (begin
+          (append! branches (nth res 0))
+          (let
+            ((ni2 (nth res 1)) (g2 (nth res 2)))
+            (if
+              (= (rx-char-at pat ni2) "|")
+              (rx-parse-alt-branches pat ni2 g2 branches)
+              (list
+                (list (dict "__t__" "alt" "__branches__" branches))
+                ni2
+                g2))))))))
+
+;; parse quantifier suffix, returns (node new-i)
+(define
+  rx-parse-quant
+  (fn
+    (pat i node)
+    (let
+      ((c (rx-char-at pat i)))
+      (cond
+        ((= c "*")
+          (let
+            ((lazy? (= (rx-char-at pat (+ i 1)) "?")))
+            (list
+              (dict
+                "__t__"
+                "quant"
+                "__node__"
+                node
+                "__min__"
+                0
+                "__max__"
+                -1
+                "__lazy__"
+                lazy?)
+              (if lazy? (+ i 2) (+ i 1)))))
+        ((= c "+")
+          (let
+            ((lazy? (= (rx-char-at pat (+ i 1)) "?")))
+            (list
+              (dict
+                "__t__"
+                "quant"
+                "__node__"
+                node
+                "__min__"
+                1
+                "__max__"
+                -1
+                "__lazy__"
+                lazy?)
+              (if lazy? (+ i 2) (+ i 1)))))
+        ((= c "?")
+          (let
+            ((lazy? (= (rx-char-at pat (+ i 1)) "?")))
+            (list
+              (dict
+                "__t__"
+                "quant"
+                "__node__"
+                node
+                "__min__"
+                0
+                "__max__"
+                1
+                "__lazy__"
+                lazy?)
+              (if lazy? (+ i 2) (+ i 1)))))
+        ((= c "{")
+          (let
+            ((mres (rx-parse-int pat (+ i 1) 0)))
+            (let
+              ((mn (nth mres 0)) (mi (nth mres 1)))
+              (let
+                ((sep (rx-char-at pat mi)))
+                (cond
+                  ((= sep "}")
+                    (let
+                      ((lazy? (= (rx-char-at pat (+ mi 1)) "?")))
+                      (list
+                        (dict
+                          "__t__"
+                          "quant"
+                          "__node__"
+                          node
+                          "__min__"
+                          mn
+                          "__max__"
+                          mn
+                          "__lazy__"
+                          lazy?)
+                        (if lazy? (+ mi 2) (+ mi 1)))))
+                  ((= sep ",")
+                    (let
+                      ((c2 (rx-char-at pat (+ mi 1))))
+                      (if
+                        (= c2 "}")
+                        (let
+                          ((lazy? (= (rx-char-at pat (+ mi 2)) "?")))
+                          (list
+                            (dict
+                              "__t__"
+                              "quant"
+                              "__node__"
+                              node
+                              "__min__"
+                              mn
+                              "__max__"
+                              -1
+                              "__lazy__"
+                              lazy?)
+                            (if lazy? (+ mi 3) (+ mi 2))))
+                        (let
+                          ((mxres (rx-parse-int pat (+ mi 1) 0)))
+                          (let
+                            ((mx (nth mxres 0)) (mxi (nth mxres 1)))
+                            (let
+                              ((lazy? (= (rx-char-at pat (+ mxi 1)) "?")))
+                              (list
+                                (dict
+                                  "__t__"
+                                  "quant"
+                                  "__node__"
+                                  node
+                                  "__min__"
+                                  mn
+                                  "__max__"
+                                  mx
+                                  "__lazy__"
+                                  lazy?)
+                                (if lazy? (+ mxi 2) (+ mxi 1)))))))))
+                  (else (list node i)))))))
+        (else (list node i))))))
+
+;; parse one atom, returns (node new-i new-ds)
+(define
+  rx-parse-atom
+  (fn
+    (pat i ds)
+    (let
+      ((c (rx-char-at pat i)))
+      (cond
+        ((= c ".") (list (dict "__t__" "any") (+ i 1) ds))
+        ((= c "^") (list (dict "__t__" "anchor-start") (+ i 1) ds))
+        ((= c "$") (list (dict "__t__" "anchor-end") (+ i 1) ds))
+        ((= c "\\")
+          (let
+            ((esc (rx-parse-escape pat (+ i 1))))
+            (list (nth esc 0) (nth esc 1) ds)))
+        ((= c "[")
+          (let
+            ((neg? (= (rx-char-at pat (+ i 1)) "^")))
+            (let
+              ((start (if neg? (+ i 2) (+ i 1))) (items (list)))
+              (let
+                ((res (rx-parse-class-items pat start items)))
+                (let
+                  ((ci (nth res 1)))
+                  (list
+                    (dict
+                      "__t__"
+                      "char-class"
+                      "__neg__"
+                      neg?
+                      "__items__"
+                      items)
+                    (+ ci 1)
+                    ds))))))
+        ((= c "(")
+          (let
+            ((c2 (rx-char-at pat (+ i 1))))
+            (if
+              (and (= c2 "?") (= (rx-char-at pat (+ i 2)) ":"))
+              (let
+                ((inner-nodes (list))
+                  (inner-ds
+                    (dict "groups" (get ds "groups") "nodes" inner-nodes)))
+                (let
+                  ((res (rx-parse-seq pat (+ i 3) ")" inner-ds)))
+                  (list
+                    (dict "__t__" "ncgroup" "__nodes__" (nth res 0))
+                    (+ (nth res 1) 1)
+                    (dict "groups" (nth res 2) "nodes" (get ds "nodes")))))
+              (let
+                ((gidx (+ (get ds "groups") 1)) (inner-nodes (list)))
+                (let
+                  ((inner-ds (dict "groups" gidx "nodes" inner-nodes)))
+                  (let
+                    ((res (rx-parse-seq pat (+ i 1) ")" inner-ds)))
+                    (list
+                      (dict
+                        "__t__"
+                        "group"
+                        "__idx__"
+                        gidx
+                        "__nodes__"
+                        (nth res 0))
+                      (+ (nth res 1) 1)
+                      (dict "groups" (nth res 2) "nodes" (get ds "nodes")))))))))
+        (else (list (dict "__t__" "literal" "__c__" c) (+ i 1) ds))))))
+
+;; top-level compile
+(define
+  rx-compile
+  (fn
+    (pattern)
+    (let
+      ((nodes (list)) (ds (dict "groups" 0 "nodes" nodes)))
+      (let
+        ((res (rx-parse-seq pattern 0 "" ds)))
+        (dict "nodes" (nth res 0) "ngroups" (nth res 2))))))
+
+;; ── Matcher ───────────────────────────────────────────────────────
+
+;; Match a char-class item against character c
+(define
+  rx-item-matches?
+  (fn
+    (item c ci?)
+    (let
+      ((kind (get item "kind")))
+      (cond
+        ((= kind "lit") (rx-char-eq? c (get item "c") ci?))
+        ((= kind "range")
+          (let
+            ((lo (if ci? (rx-downcase-char (get item "lo")) (get item "lo")))
+              (hi
+                (if ci? (rx-downcase-char (get item "hi")) (get item "hi")))
+              (dc (if ci? (rx-downcase-char c) c)))
+            (and
+              (>= (char-code dc) (char-code lo))
+              (<= (char-code dc) (char-code hi)))))
+        ((= kind "class-d")
+          (let ((m (rx-digit? c))) (if (get item "neg") (not m) m)))
+        ((= kind "class-w")
+          (let ((m (rx-word? c))) (if (get item "neg") (not m) m)))
+        ((= kind "class-s")
+          (let ((m (rx-space? c))) (if (get item "neg") (not m) m)))
+        (else false)))))
+
+(define
+  rx-class-items-any?
+  (fn
+    (items c ci?)
+    (if
+      (empty? items)
+      false
+      (if
+        (rx-item-matches? (first items) c ci?)
+        true
+        (rx-class-items-any? (rest items) c ci?)))))
+
+(define
+  rx-class-matches?
+  (fn
+    (node c ci?)
+    (let
+      ((neg? (get node "__neg__")) (items (get node "__items__")))
+      (let
+        ((hit (rx-class-items-any? items c ci?)))
+        (if neg? (not hit) hit)))))
+
+;; Word boundary check
+(define
+  rx-is-word-boundary?
+  (fn
+    (s i slen)
+    (let
+      ((before (if (> i 0) (rx-word? (char-at s (- i 1))) false))
+        (after (if (< i slen) (rx-word? (char-at s i)) false)))
+      (not (= before after)))))
+
+;; ── Core matcher ──────────────────────────────────────────────────
+;;
+;; rx-match-nodes : nodes s i slen ci? mi? groups → end-pos or -1
+;;
+;; Matches `nodes` starting at position `i` in string `s`.
+;; Returns the position after the last character consumed, or -1 on failure.
+;; Mutates `groups` dict to record captures.
+
+(define
+  rx-match-nodes
+  (fn
+    (nodes s i slen ci? mi? groups)
+    (if
+      (empty? nodes)
+      i
+      (let
+        ((node (first nodes)) (rest-nodes (rest nodes)))
+        (let
+          ((t (get node "__t__")))
+          (cond
+            ((= t "literal")
+              (if
+                (and
+                  (< i slen)
+                  (rx-char-eq? (char-at s i) (get node "__c__") ci?))
+                (rx-match-nodes rest-nodes s (+ i 1) slen ci? mi? groups)
+                -1))
+            ((= t "any")
+              (if
+                (and (< i slen) (not (rx-newline? (char-at s i))))
+                (rx-match-nodes rest-nodes s (+ i 1) slen ci? mi? groups)
+                -1))
+            ((= t "class-d")
+              (let
+                ((m (and (< i slen) (rx-digit? (char-at s i)))))
+                (if
+                  (if (get node "__neg__") (not m) m)
+                  (rx-match-nodes rest-nodes s (+ i 1) slen ci? mi? groups)
+                  -1)))
+            ((= t "class-w")
+              (let
+                ((m (and (< i slen) (rx-word? (char-at s i)))))
+                (if
+                  (if (get node "__neg__") (not m) m)
+                  (rx-match-nodes rest-nodes s (+ i 1) slen ci? mi? groups)
+                  -1)))
+            ((= t "class-s")
+              (let
+                ((m (and (< i slen) (rx-space? (char-at s i)))))
+                (if
+                  (if (get node "__neg__") (not m) m)
+                  (rx-match-nodes rest-nodes s (+ i 1) slen ci? mi? groups)
+                  -1)))
+            ((= t "char-class")
+              (if
+                (and (< i slen) (rx-class-matches? node (char-at s i) ci?))
+                (rx-match-nodes rest-nodes s (+ i 1) slen ci? mi? groups)
+                -1))
+            ((= t "anchor-start")
+              (if
+                (or
+                  (= i 0)
+                  (and mi? (rx-newline? (rx-char-at s (- i 1)))))
+                (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+                -1))
+            ((= t "anchor-end")
+              (if
+                (or (= i slen) (and mi? (rx-newline? (rx-char-at s i))))
+                (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+                -1))
+            ((= t "anchor-word")
+              (if
+                (rx-is-word-boundary? s i slen)
+                (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+                -1))
+            ((= t "anchor-nonword")
+              (if
+                (not (rx-is-word-boundary? s i slen))
+                (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+                -1))
+            ((= t "group")
+              (let
+                ((gidx (get node "__idx__"))
+                  (inner (get node "__nodes__")))
+                (let
+                  ((g-end (rx-match-nodes inner s i slen ci? mi? groups)))
+                  (if
+                    (>= g-end 0)
+                    (begin
+                      (dict-set!
+                        groups
+                        (js-to-string gidx)
+                        (substring s i g-end))
+                      (let
+                        ((final-end (rx-match-nodes rest-nodes s g-end slen ci? mi? groups)))
+                        (if
+                          (>= final-end 0)
+                          final-end
+                          (begin
+                            (dict-set! groups (js-to-string gidx) nil)
+                            -1))))
+                    -1))))
+            ((= t "ncgroup")
+              (let
+                ((inner (get node "__nodes__")))
+                (rx-match-nodes
+                  (append inner rest-nodes)
+                  s
+                  i
+                  slen
+                  ci?
+                  mi?
+                  groups)))
+            ((= t "alt")
+              (let
+                ((branches (get node "__branches__")))
+                (rx-try-branches branches rest-nodes s i slen ci? mi? groups)))
+            ((= t "quant")
+              (let
+                ((inner-node (get node "__node__"))
+                  (mn (get node "__min__"))
+                  (mx (get node "__max__"))
+                  (lazy? (get node "__lazy__")))
+                (if
+                  lazy?
+                  (rx-quant-lazy
+                    inner-node
+                    mn
+                    mx
+                    rest-nodes
+                    s
+                    i
+                    slen
+                    ci?
+                    mi?
+                    groups
+                    0)
+                  (rx-quant-greedy
+                    inner-node
+                    mn
+                    mx
+                    rest-nodes
+                    s
+                    i
+                    slen
+                    ci?
+                    mi?
+                    groups
+                    0))))
+            (else -1)))))))
+
+(define
+  rx-try-branches
+  (fn
+    (branches rest-nodes s i slen ci? mi? groups)
+    (if
+      (empty? branches)
+      -1
+      (let
+        ((res (rx-match-nodes (append (first branches) rest-nodes) s i slen ci? mi? groups)))
+        (if
+          (>= res 0)
+          res
+          (rx-try-branches (rest branches) rest-nodes s i slen ci? mi? groups))))))
+
+;; Greedy: expand as far as possible, then try rest from the longest match
+;; Strategy: recurse forward (extend first); only try rest when extension fails
+(define
+  rx-quant-greedy
+  (fn
+    (inner-node mn mx rest-nodes s i slen ci? mi? groups count)
+    (let
+      ((can-extend (and (< i slen) (or (= mx -1) (< count mx)))))
+      (if
+        can-extend
+        (let
+          ((ni (rx-match-one inner-node s i slen ci? mi? groups)))
+          (if
+            (>= ni 0)
+            (let
+              ((res (rx-quant-greedy inner-node mn mx rest-nodes s ni slen ci? mi? groups (+ count 1))))
+              (if
+                (>= res 0)
+                res
+                (if
+                  (>= count mn)
+                  (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+                  -1)))
+            (if
+              (>= count mn)
+              (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+              -1)))
+        (if
+          (>= count mn)
+          (rx-match-nodes rest-nodes s i slen ci? mi? groups)
+          -1)))))
+
+;; Lazy: try rest first, extend only if rest fails
+(define
+  rx-quant-lazy
+  (fn
+    (inner-node mn mx rest-nodes s i slen ci? mi? groups count)
+    (if
+      (>= count mn)
+      (let
+        ((res (rx-match-nodes rest-nodes s i slen ci? mi? groups)))
+        (if
+          (>= res 0)
+          res
+          (if
+            (and (< i slen) (or (= mx -1) (< count mx)))
+            (let
+              ((ni (rx-match-one inner-node s i slen ci? mi? groups)))
+              (if
+                (>= ni 0)
+                (rx-quant-lazy
+                  inner-node
+                  mn
+                  mx
+                  rest-nodes
+                  s
+                  ni
+                  slen
+                  ci?
+                  mi?
+                  groups
+                  (+ count 1))
+                -1))
+            -1)))
+      (if
+        (< i slen)
+        (let
+          ((ni (rx-match-one inner-node s i slen ci? mi? groups)))
+          (if
+            (>= ni 0)
+            (rx-quant-lazy
+              inner-node
+              mn
+              mx
+              rest-nodes
+              s
+              ni
+              slen
+              ci?
+              mi?
+              groups
+              (+ count 1))
+            -1))
+        -1))))
+
+;; Match a single node at position i, return new pos or -1
+(define
+  rx-match-one
+  (fn
+    (node s i slen ci? mi? groups)
+    (rx-match-nodes (list node) s i slen ci? mi? groups)))
+
+;; ── Engine entry points ───────────────────────────────────────────
+
+;; Try matching at exactly position i. Returns result dict or nil.
+(define
+  rx-try-at
+  (fn
+    (compiled s i slen ci? mi?)
+    (let
+      ((nodes (get compiled "nodes")) (ngroups (get compiled "ngroups")))
+      (let
+        ((groups (dict)))
+        (let
+          ((end (rx-match-nodes nodes s i slen ci? mi? groups)))
+          (if
+            (>= end 0)
+            (dict "start" i "end" end "groups" groups "ngroups" ngroups)
+            nil))))))
+
+;; Find first match scanning from search-start.
+(define
+  rx-find-from
+  (fn
+    (compiled s search-start slen ci? mi?)
+    (if
+      (> search-start slen)
+      nil
+      (let
+        ((res (rx-try-at compiled s search-start slen ci? mi?)))
+        (if
+          res
+          res
+          (rx-find-from compiled s (+ search-start 1) slen ci? mi?))))))
+
+;; Build exec result dict from raw match result
+(define
+  rx-build-exec-result
+  (fn
+    (s match-res)
+    (let
+      ((start (get match-res "start"))
+        (end (get match-res "end"))
+        (groups (get match-res "groups"))
+        (ngroups (get match-res "ngroups")))
+      (let
+        ((matched (substring s start end))
+          (caps (rx-build-captures groups ngroups 1)))
+        (dict "match" matched "index" start "input" s "groups" caps)))))
+
+(define
+  rx-build-captures
+  (fn
+    (groups ngroups idx)
+    (if
+      (> idx ngroups)
+      (list)
+      (let
+        ((cap (get groups (js-to-string idx))))
+        (cons
+          (if (= cap nil) :js-undefined cap)
+          (rx-build-captures groups ngroups (+ idx 1)))))))
+
+;; ── Public interface ──────────────────────────────────────────────
+
+;; Lazy compile: build NFA on first use, cache under "__compiled__"
+(define
+  rx-ensure-compiled!
+  (fn
+    (rx)
+    (if
+      (dict-has? rx "__compiled__")
+      (get rx "__compiled__")
+      (let
+        ((c (rx-compile (get rx "source"))))
+        (begin (dict-set! rx "__compiled__" c) c)))))
+
+(define
+  rx-test
+  (fn
+    (rx s)
+    (let
+      ((compiled (rx-ensure-compiled! rx))
+        (ci? (get rx "ignoreCase"))
+        (mi? (get rx "multiline"))
+        (slen (len s)))
+      (let
+        ((start (if (get rx "global") (let ((li (get rx "lastIndex"))) (if (number? li) li 0)) 0)))
+        (let
+          ((res (rx-find-from compiled s start slen ci? mi?)))
+          (if
+            (get rx "global")
+            (begin
+              (dict-set! rx "lastIndex" (if res (get res "end") 0))
+              (if res true false))
+            (if res true false)))))))
+
+(define
+  rx-exec
+  (fn
+    (rx s)
+    (let
+      ((compiled (rx-ensure-compiled! rx))
+        (ci? (get rx "ignoreCase"))
+        (mi? (get rx "multiline"))
+        (slen (len s)))
+      (let
+        ((start (if (get rx "global") (let ((li (get rx "lastIndex"))) (if (number? li) li 0)) 0)))
+        (let
+          ((res (rx-find-from compiled s start slen ci? mi?)))
+          (if
+            res
+            (begin
+              (when
+                (get rx "global")
+                (dict-set! rx "lastIndex" (get res "end")))
+              (rx-build-exec-result s res))
+            (begin
+              (when (get rx "global") (dict-set! rx "lastIndex" 0))
+              nil)))))))
+
+;; match-all for String.prototype.matchAll
+(define
+  js-regex-match-all
+  (fn
+    (rx s)
+    (let
+      ((compiled (rx-ensure-compiled! rx))
+        (ci? (get rx "ignoreCase"))
+        (mi? (get rx "multiline"))
+        (slen (len s))
+        (results (list)))
+      (rx-match-all-loop compiled s 0 slen ci? mi? results))))
+
+(define
+  rx-match-all-loop
+  (fn
+    (compiled s i slen ci? mi? results)
+    (if
+      (> i slen)
+      results
+      (let
+        ((res (rx-find-from compiled s i slen ci? mi?)))
+        (if
+          res
+          (begin
+            (append! results (rx-build-exec-result s res))
+            (let
+              ((next (get res "end")))
+              (rx-match-all-loop
+                compiled
+                s
+                (if (= next i) (+ i 1) next)
+                slen
+                ci?
+                mi?
+                results)))
+          results)))))
+
+;; ── Install platform ──────────────────────────────────────────────
+
+(js-regex-platform-override! "test" rx-test)
+(js-regex-platform-override! "exec" rx-exec)
diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index e1021cc2..1872b3e9 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -2032,7 +2032,15 @@
           (&rest args)
           (cond
             ((= (len args) 0) nil)
-            ((js-regex? (nth args 0)) (js-regex-stub-exec (nth args 0) s))
+            ((js-regex? (nth args 0))
+              (let
+                ((rx (nth args 0)))
+                (let
+                  ((impl (get __js_regex_platform__ "exec")))
+                  (if
+                    (js-undefined? impl)
+                    (js-regex-stub-exec rx s)
+                    (impl rx s)))))
             (else
               (let
                 ((needle (js-to-string (nth args 0))))
@@ -2041,7 +2049,7 @@
                   (if
                     (= idx -1)
                     nil
-                    (let ((res (list))) (append! res needle) res))))))))
+                    (let ((res (list))) (begin (append! res needle) res)))))))))
       ((= name "at")
         (fn
           (i)
@@ -2099,6 +2107,20 @@
       ((= name "toWellFormed") (fn () s))
       (else js-undefined))))
 
+(define __js_tdz_sentinel__ (dict "__tdz__" true))
+
+(define js-tdz? (fn (v) (and (dict? v) (dict-has? v "__tdz__"))))
+
+(define
+  js-tdz-check
+  (fn
+    (name val)
+    (if
+      (js-tdz? val)
+      (raise
+        (TypeError (str "Cannot access '" name "' before initialization")))
+      val)))
+
 (define
   js-string-slice
   (fn
diff --git a/lib/js/test.sh b/lib/js/test.sh
index de6caea5..80cb135a 100755
--- a/lib/js/test.sh
+++ b/lib/js/test.sh
@@ -33,6 +33,8 @@ cat > "$TMPFILE" << 'EPOCHS'
 (load "lib/js/transpile.sx")
 (epoch 5)
 (load "lib/js/runtime.sx")
+(epoch 6)
+(load "lib/js/regex.sx")
 
 ;; ── Phase 0: stubs still behave ─────────────────────────────────
 (epoch 10)
@@ -1323,6 +1325,108 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3505)
 (eval "(js-eval \"var a = {length: 3, 0: 10, 1: 20, 2: 30}; var sum = 0; Array.prototype.forEach.call(a, function(x){sum += x;}); sum\")")
 
+;; ── Phase 12: Regex engine ────────────────────────────────────────
+;; Platform is installed (test key is a function, not undefined)
+(epoch 5000)
+(eval "(js-undefined? (get __js_regex_platform__ \"test\"))")
+(epoch 5001)
+(eval "(js-eval \"/foo/.test('hi foo bar')\")")
+(epoch 5002)
+(eval "(js-eval \"/foo/.test('hi bar')\")")
+;; Case-insensitive flag
+(epoch 5003)
+(eval "(js-eval \"/FOO/i.test('hello foo world')\")")
+;; Anchors
+(epoch 5004)
+(eval "(js-eval \"/^hello/.test('hello world')\")")
+(epoch 5005)
+(eval "(js-eval \"/^hello/.test('say hello')\")")
+(epoch 5006)
+(eval "(js-eval \"/world$/.test('hello world')\")")
+;; Character classes
+(epoch 5007)
+(eval "(js-eval \"/\\\\d+/.test('abc 123')\")")
+(epoch 5008)
+(eval "(js-eval \"/\\\\w+/.test('hello')\")")
+(epoch 5009)
+(eval "(js-eval \"/[abc]/.test('dog')\")")
+(epoch 5010)
+(eval "(js-eval \"/[abc]/.test('cat')\")")
+;; Quantifiers
+(epoch 5011)
+(eval "(js-eval \"/a*b/.test('b')\")")
+(epoch 5012)
+(eval "(js-eval \"/a+b/.test('b')\")")
+(epoch 5013)
+(eval "(js-eval \"/a{2,3}/.test('aa')\")")
+(epoch 5014)
+(eval "(js-eval \"/a{2,3}/.test('a')\")")
+;; Dot
+(epoch 5015)
+(eval "(js-eval \"/h.llo/.test('hello')\")")
+(epoch 5016)
+(eval "(js-eval \"/h.llo/.test('hllo')\")")
+;; exec result
+(epoch 5017)
+(eval "(js-eval \"var m = /foo(\\\\w+)/.exec('foobar'); m.match\")")
+(epoch 5018)
+(eval "(js-eval \"var m = /foo(\\\\w+)/.exec('foobar'); m.index\")")
+(epoch 5019)
+(eval "(js-eval \"var m = /foo(\\\\w+)/.exec('foobar'); m.groups[0]\")")
+;; Alternation
+(epoch 5020)
+(eval "(js-eval \"/cat|dog/.test('I have a dog')\")")
+(epoch 5021)
+(eval "(js-eval \"/cat|dog/.test('I have a fish')\")")
+;; Non-capturing group
+(epoch 5022)
+(eval "(js-eval \"/(?:foo)+/.test('foofoo')\")")
+;; Negated char class
+(epoch 5023)
+(eval "(js-eval \"/[^abc]/.test('d')\")")
+(epoch 5024)
+(eval "(js-eval \"/[^abc]/.test('a')\")")
+;; Range inside char class
+(epoch 5025)
+(eval "(js-eval \"/[a-z]+/.test('hello')\")")
+;; Word boundary
+(epoch 5026)
+(eval "(js-eval \"/\\\\bword\\\\b/.test('a word here')\")")
+(epoch 5027)
+(eval "(js-eval \"/\\\\bword\\\\b/.test('password')\")")
+;; Lazy quantifier
+(epoch 5028)
+(eval "(js-eval \"var m = /a+?/.exec('aaa'); m.match\")")
+;; Global flag exec
+(epoch 5029)
+(eval "(js-eval \"var r=/\\\\d+/g; r.exec('a1b2'); r.exec('a1b2').match\")")
+;; String.prototype.match with regex
+(epoch 5030)
+(eval "(js-eval \"'hello world'.match(/\\\\w+/).match\")")
+;; String.prototype.search
+(epoch 5031)
+(eval "(js-eval \"'hello world'.search(/world/)\")")
+;; String.prototype.replace with regex
+(epoch 5032)
+(eval "(js-eval \"'hello world'.replace(/world/, 'there')\")")
+;; multiline anchor
+(epoch 5033)
+(eval "(js-eval \"/^bar/m.test('foo\\nbar')\")")
+
+;; ── Phase 13: let/const TDZ infrastructure ───────────────────────
+;; The TDZ sentinel and checker are defined in runtime.sx.
+;; let/const bindings work normally after initialization.
+(epoch 5100)
+(eval "(js-eval \"let x = 5; x\")")
+(epoch 5101)
+(eval "(js-eval \"const y = 42; y\")")
+;; TDZ sentinel exists and is detectable
+(epoch 5102)
+(eval "(js-tdz? __js_tdz_sentinel__)")
+;; js-tdz-check passes through non-sentinel values
+(epoch 5103)
+(eval "(js-tdz-check \"x\" 42)")
+
 EPOCHS
 
 
@@ -2042,6 +2146,48 @@ check 3503 "indexOf.call arrLike"          '1'
 check 3504 "filter.call arrLike"           '"2,3"'
 check 3505 "forEach.call arrLike sum"      '60'
 
+# ── Phase 12: Regex engine ────────────────────────────────────────
+check 5000 "regex platform installed"           'false'
+check 5001 "/foo/ matches"                      'true'
+check 5002 "/foo/ no match"                     'false'
+check 5003 "/FOO/i case-insensitive"            'true'
+check 5004 "/^hello/ anchor match"              'true'
+check 5005 "/^hello/ anchor no-match"           'false'
+check 5006 "/world$/ end anchor"                'true'
+check 5007 "/\\d+/ digit class"                 'true'
+check 5008 "/\\w+/ word class"                  'true'
+check 5009 "/[abc]/ class no-match"             'false'
+check 5010 "/[abc]/ class match"                'true'
+check 5011 "/a*b/ zero-or-more"                 'true'
+check 5012 "/a+b/ one-or-more no-match"         'false'
+check 5013 "/a{2,3}/ quant match"               'true'
+check 5014 "/a{2,3}/ quant no-match"            'false'
+check 5015 "dot matches any"                    'true'
+check 5016 "dot requires char"                  'false'
+check 5017 "exec match string"                  '"foobar"'
+check 5018 "exec match index"                   '0'
+check 5019 "exec capture group"                 '"bar"'
+check 5020 "alternation cat|dog match"          'true'
+check 5021 "alternation cat|dog no-match"       'false'
+check 5022 "non-capturing group"                'true'
+check 5023 "negated class match"                'true'
+check 5024 "negated class no-match"             'false'
+check 5025 "range [a-z]+"                       'true'
+check 5026 "word boundary match"                'true'
+check 5027 "word boundary no-match"             'false'
+check 5028 "lazy quantifier"                    '"a"'
+check 5029 "global exec advances"               '"2"'
+check 5030 "String.match regex"                 '"hello"'
+check 5031 "String.search regex"                '6'
+check 5032 "String.replace regex"               '"hello there"'
+check 5033 "multiline anchor"                   'true'
+
+# ── Phase 13: let/const TDZ infrastructure ───────────────────────
+check 5100 "let binding initialized"            '5'
+check 5101 "const binding initialized"          '42'
+check 5102 "TDZ sentinel is detectable"         'true'
+check 5103 "tdz-check passes non-sentinel"      '42'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"
diff --git a/lib/js/test262-runner.py b/lib/js/test262-runner.py
index 9a0807b7..0b803c37 100644
--- a/lib/js/test262-runner.py
+++ b/lib/js/test262-runner.py
@@ -798,6 +798,7 @@ class ServerSession:
         self._run_and_collect(3, '(load "lib/js/parser.sx")', timeout=60.0)
         self._run_and_collect(4, '(load "lib/js/transpile.sx")', timeout=60.0)
         self._run_and_collect(5, '(load "lib/js/runtime.sx")', timeout=60.0)
+        self._run_and_collect(50, '(load "lib/js/regex.sx")', timeout=60.0)
         # Preload the stub harness — use precomputed SX cache when available
         # (huge win: ~15s js-eval HARNESS_STUB → ~0s load precomputed .sx).
         cache_rel = _harness_cache_rel_path()