diff --git a/lib/chibi/regexp.scm b/lib/chibi/regexp.scm
index dddfbf05..0f9a655f 100644
--- a/lib/chibi/regexp.scm
+++ b/lib/chibi/regexp.scm
@@ -578,6 +578,9 @@
      (else
       (lp (cddr ls) (cons (cons (car ls) (cadr ls)) res))))))
 
+(define (every pred ls)
+  (or (null? ls) (and (pred (car ls)) (every pred (cdr ls)))))
+
 (define (char-set-sre? sre)
   (or (char? sre)
       (and (string? sre) (= 1 (string-length sre)))
@@ -587,10 +590,32 @@
                (memq (car sre)
                      '(char-set / char-range & and ~ complement - difference))
                (and (memq (car sre) '(|\|| or))
-                    (let lp ((ls (cdr sre)))
-                      (or (null? ls)
-                          (and (char-set-sre? (car ls))
-                               (lp (cdr ls))))))))))
+                    (every char-set-sre? (cdr sre)))))))
+
+(define (valid-sre? x)
+  (or (regexp? x)
+      (string? x)
+      (char-set-sre? x)
+      (and (pair? x)
+           (memq (car x)
+                 '(|\|| or : seq $ submatch *$ submatch-list ? optional
+                   * zero-or-more + one-or-more ?? non-greedy-optional
+                   *? non-greedy-zero-or-more +? non-greedy-one-or-more
+                   look-ahead neg-look-ahead look-behind neg-look-behind
+                   w/case w/nocase w/unicode w/ascii word word+))
+           (every valid-sre? (cdr x)))
+      (and (pair? x)
+           (memq (car x)
+                 '(>= -> => submatch-named *-> *=> submatch-named-list))
+           (pair? (cdr x))
+           (every valid-sre? (cddr x)))
+      (and (pair? x)
+           (memq (car x) '(** repeated **? non-greedy-repeated))
+           (pair? (cdr x))
+           (pair? (cddr x))
+           (or (not (cadr x)) (integer? (cadr x)))
+           (or (not (car (cddr x))) (integer? (car (cddr x))))
+           (every valid-sre? (cdr (cddr x))))))
 
 (define (sre->char-set sre . o)
   (let ((flags (if (pair? o) (car o) ~none)))
diff --git a/lib/chibi/regexp.sld b/lib/chibi/regexp.sld
index 09e87c70..3383ae63 100644
--- a/lib/chibi/regexp.sld
+++ b/lib/chibi/regexp.sld
@@ -1,6 +1,6 @@
 
 (define-library (chibi regexp)
-  (export regexp regexp? rx regexp->sre char-set->sre
+  (export regexp regexp? valid-sre? rx regexp->sre char-set->sre
           regexp-matches regexp-matches? regexp-search
           regexp-replace regexp-replace-all
           regexp-fold regexp-extract regexp-split