diff --git a/src/ast/rewriter/seq_derive.cpp b/src/ast/rewriter/seq_derive.cpp
index 36d1ab560..0cb766d99 100644
--- a/src/ast/rewriter/seq_derive.cpp
+++ b/src/ast/rewriter/seq_derive.cpp
@@ -519,8 +519,7 @@ namespace seq {
             m_br.mk_not(is_nullable(r1), result);
         }
         else if (re().is_to_re(r, r1)) {
-            SASSERT(u().is_seq(r1->get_sort()));
-            result = m.mk_eq(u().str.mk_empty(r1->get_sort()), r1);
+            result = is_nullable(r1);
         }
         else if (m.is_ite(r, cond, r1, r2)) {
             m_br.mk_ite(cond, is_nullable(r1), is_nullable(r2), result);
diff --git a/src/ast/rewriter/seq_derive.h b/src/ast/rewriter/seq_derive.h
index 69f38e72f..676f67f81 100644
--- a/src/ast/rewriter/seq_derive.h
+++ b/src/ast/rewriter/seq_derive.h
@@ -182,6 +182,11 @@ namespace seq {
          */
         expr_ref operator()(expr* r);
 
+        /**
+         * Nullable check: returns a Boolean expression that is true iff r accepts the empty string.
+         */
+        expr_ref nullable(expr* r) { return is_nullable(r); }
+
     };
 
 }
diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index fa200f849..3ea2b630c 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -2596,7 +2596,7 @@ expr_ref seq_rewriter::is_nullable(expr* r) {
                                << mk_pp(r, m()) << std::endl;);
     expr_ref result(m_op_cache.find(_OP_RE_IS_NULLABLE, r, nullptr, nullptr), m());
     if (!result) {
-        result = is_nullable_rec(r);
+        result = m_derive.nullable(r);
         m_op_cache.insert(_OP_RE_IS_NULLABLE, r, nullptr, nullptr, result);        
     }
     STRACE(seq_verbose, tout << "is_nullable result: "
@@ -2604,100 +2604,6 @@ expr_ref seq_rewriter::is_nullable(expr* r) {
     return result;
 }
 
-expr_ref seq_rewriter::is_nullable_rec(expr* r) {
-    SASSERT(m_util.is_re(r) || m_util.is_seq(r));
-    expr* r1 = nullptr, *r2 = nullptr, *cond = nullptr;
-    sort* seq_sort = nullptr;
-    unsigned lo = 0, hi = 0;
-    zstring s1;
-    expr_ref result(m());
-    if (re().is_concat(r, r1, r2) ||
-        re().is_intersection(r, r1, r2)) { 
-        m_br.mk_and(is_nullable(r1), is_nullable(r2), result);
-    }
-    else if (re().is_union(r, r1, r2) || re().is_antimirov_union(r, r1, r2)) {
-        m_br.mk_or(is_nullable(r1), is_nullable(r2), result);
-    }
-    else if (re().is_diff(r, r1, r2)) {
-        m_br.mk_not(is_nullable(r2), result);
-        m_br.mk_and(result, is_nullable(r1), result);
-    }
-    else if (re().is_star(r) || 
-        re().is_opt(r) ||
-        re().is_full_seq(r) ||
-        re().is_epsilon(r) ||
-        (re().is_loop(r, r1, lo) && lo == 0) || 
-        (re().is_loop(r, r1, lo, hi) && lo == 0)) {
-        result = m().mk_true();
-    }
-    else if (re().is_full_char(r) ||
-        re().is_empty(r) ||
-        re().is_of_pred(r) ||
-        re().is_range(r)) {
-        result = m().mk_false();
-    }
-    else if (re().is_plus(r, r1) ||
-        (re().is_loop(r, r1, lo) && lo > 0) ||
-        (re().is_loop(r, r1, lo, hi) && lo > 0) ||
-        (re().is_reverse(r, r1))) {
-        result = is_nullable(r1);
-    }
-    else if (re().is_complement(r, r1)) {
-        m_br.mk_not(is_nullable(r1), result);
-    }
-    else if (re().is_to_re(r, r1)) {        
-        result = is_nullable(r1);
-    }
-    else if (m().is_ite(r, cond, r1, r2)) {
-        m_br.mk_ite(cond, is_nullable(r1), is_nullable(r2), result);
-    }
-    else if (m_util.is_re(r, seq_sort)) {
-        result = is_nullable_symbolic_regex(r, seq_sort);
-    }
-    else if (str().is_concat(r, r1, r2)) {
-        m_br.mk_and(is_nullable(r1), is_nullable(r2), result);
-    }
-    else if (str().is_empty(r)) {
-        result = m().mk_true();
-    }
-    else if (str().is_unit(r)) {
-        result = m().mk_false();
-    }
-    else if (str().is_string(r, s1)) {
-        result = m().mk_bool_val(s1.length() == 0);
-    }
-    else {
-        SASSERT(m_util.is_seq(r));
-        result = m().mk_eq(str().mk_empty(r->get_sort()), r);
-    }
-    return result;
-}
-
-expr_ref seq_rewriter::is_nullable_symbolic_regex(expr* r, sort* seq_sort) {
-    SASSERT(m_util.is_re(r));
-    expr* elem = nullptr, *r1 = r, * r2 = nullptr, * s = nullptr;
-    expr_ref elems(str().mk_empty(seq_sort), m());
-    expr_ref result(m());
-    while (re().is_derivative(r1, elem, r2)) {
-        if (str().is_empty(elems))
-            elems = str().mk_unit(elem);
-        else
-            elems = str().mk_concat(str().mk_unit(elem), elems);
-        r1 = r2;
-    }
-    if (re().is_to_re(r1, s)) {
-        // r is nullable  
-        // iff after taking the derivatives the remaining sequence is empty 
-        // iff the inner sequence equals to the sequence of derivative elements in reverse
-        result = m().mk_eq(elems, s);
-        return result;
-    }
-    // the default case when either r is not a derivative
-    // or when the nested derivatives are not applied to a sequence
-    result = re().mk_in_re(str().mk_empty(seq_sort), r);
-    return result;
-}
-
 /*
     Push reverse inwards (whenever possible).
 */
diff --git a/src/ast/rewriter/seq_rewriter.h b/src/ast/rewriter/seq_rewriter.h
index 8b4bfd5b7..2e05bf3c7 100644
--- a/src/ast/rewriter/seq_rewriter.h
+++ b/src/ast/rewriter/seq_rewriter.h
@@ -179,7 +179,6 @@ class seq_rewriter {
     void replace_all_subvectors(expr_ref_vector const& as, expr_ref_vector const& bs, expr* c, expr_ref_vector& result);
 
     // Calculate derivative, memoized and enforcing a normal form
-    expr_ref is_nullable_rec(expr* r);
     expr_ref mk_der_op(decl_kind k, expr* a, expr* b);
     expr_ref mk_der_op_rec(decl_kind k, expr* a, expr* b);
     expr_ref mk_der_concat(expr* a, expr* b);
@@ -189,8 +188,6 @@ class seq_rewriter {
     expr_ref mk_der_cond(expr* cond, expr* ele, sort* seq_sort);
     expr_ref mk_der_antimirov_union(expr* r1, expr* r2);
     bool ite_bdds_compatible(expr* a, expr* b);
-    /* if r has the form deriv(en..deriv(e1,to_re(s))..) returns 's = [e1..en]' else returns '() in r'*/
-    expr_ref is_nullable_symbolic_regex(expr* r, sort* seq_sort);
     #ifdef Z3DEBUG
     bool check_deriv_normal_form(expr* r, int level = 3);
     #endif