misc edits of work in progress

src/ast/rewriter/CMakeLists.txt

@@ -38,6 +38,7 @@ z3_add_component(rewriter
     range_predicate.cpp
     range_predicate_translator.cpp
     recfun_rewriter.cpp
+    regex_range_collapse.cpp
     rewriter.cpp
     seq_axioms.cpp
     seq_eq_solver.cpp

src/ast/rewriter/regex_range_collapse.cpp

@@ -0,0 +1,145 @@
+/*++
+Copyright (c) 2026 Microsoft Corporation
+
+Module Name:
+
+    regex_range_collapse.cpp
+
+Abstract:
+
+    Implementation of regex <-> range_predicate translation for the
+    boolean-combination-of-ranges fragment. See header for the recognized
+    grammar and the canonical regex AST emitted by materialization.
+
+Authors:
+
+    Margus Veanes (veanes) 2026
+
+--*/
+
+#include "ast/rewriter/regex_range_collapse.h"
+
+namespace seq {
+
+    bool regex_to_range_predicate(seq_util& u, expr* r, range_predicate& out) {
+        unsigned const max_char = u.max_char();
+        auto& re = u.re;
+
+        if (re.is_empty(r)) {
+            out = range_predicate::empty(max_char);
+            return true;
+        }
+        if (re.is_full_char(r)) {
+            out = range_predicate::top(max_char);
+            return true;
+        }
+        unsigned lo = 0, hi = 0;
+        expr* lo_e = nullptr;
+        expr* hi_e = nullptr;
+        if (re.is_range(r, lo_e, hi_e)) {
+            auto extract_char = [&](expr* e, unsigned& c) -> bool {
+                if (u.is_const_char(e, c)) return true;
+                expr* inner = nullptr;
+                if (u.str.is_unit(e, inner) && u.is_const_char(inner, c)) return true;
+                zstring s;
+                if (u.str.is_string(e, s) && s.length() == 1) {
+                    c = s[0];
+                    return true;
+                }
+                return false;
+            };
+            if (!extract_char(lo_e, lo) || !extract_char(hi_e, hi))
+                return false;
+            // Empty/inverted range [lo > hi] is the empty regex.
+            if (lo > hi) {
+                out = range_predicate::empty(max_char);
+                return true;
+            }
+            out = range_predicate::range(lo, hi, max_char);
+            return true;
+        }
+        expr* a = nullptr;
+        expr* b = nullptr;
+        if (re.is_union(r, a, b)) {
+            range_predicate pa(max_char), pb(max_char);
+            if (!regex_to_range_predicate(u, a, pa)) return false;
+            if (!regex_to_range_predicate(u, b, pb)) return false;
+            out = pa | pb;
+            return true;
+        }
+        if (re.is_intersection(r, a, b)) {
+            range_predicate pa(max_char), pb(max_char);
+            if (!regex_to_range_predicate(u, a, pa)) return false;
+            if (!regex_to_range_predicate(u, b, pb)) return false;
+            out = pa & pb;
+            return true;
+        }
+        if (re.is_diff(r, a, b)) {
+            range_predicate pa(max_char), pb(max_char);
+            if (!regex_to_range_predicate(u, a, pa)) return false;
+            if (!regex_to_range_predicate(u, b, pb)) return false;
+            out = pa - pb;
+            return true;
+        }
+        // NOTE: re.complement is intentionally NOT handled here.
+        //   re.complement is the SEQUENCE-level complement: its language
+        //   includes the empty string, strings of length >= 2, and any
+        //   length-1 string outside the operand.  A character-class
+        //   range_predicate can only describe a set of length-1 strings,
+        //   so collapsing re.complement(R) to ~R (character-level
+        //   complement) would change semantics whenever R is wrapped in
+        //   any sequence-level context (e.g. re.diff at the top level,
+        //   or membership tests).  De-Morgan equivalences and the
+        //   special cases re.complement(re.empty) / re.complement(re.full)
+        //   are already handled directly in seq_rewriter::mk_re_complement.
+        return false;
+    }
+
+    static expr_ref mk_unit_string_from_char(seq_util& u, unsigned c) {
+        return expr_ref(u.str.mk_string(zstring(c)), u.get_manager());
+    }
+
+    static expr_ref mk_single_range_regex(seq_util& u, unsigned lo, unsigned hi, sort* re_sort) {
+        ast_manager& m = u.get_manager();
+        if (lo == 0 && hi == u.max_char())
+            return expr_ref(u.re.mk_full_char(re_sort), m);
+        // Use the canonical unit-character form (seq.unit (Char N)) for
+        // range bounds.  This matches the shape used elsewhere in
+        // seq_rewriter and avoids creating duplicate AST nodes with
+        // different ids for semantically identical ranges.
+        expr_ref slo(u.str.mk_unit(u.str.mk_char(lo)), m);
+        expr_ref shi(u.str.mk_unit(u.str.mk_char(hi)), m);
+        return expr_ref(u.re.mk_range(slo, shi), m);
+    }
+
+    expr_ref range_predicate_to_regex(seq_util& u, range_predicate const& p, sort* seq_sort) {
+        ast_manager& m = u.get_manager();
+        sort* re_sort = u.re.mk_re(seq_sort);
+        if (p.is_empty())
+            return expr_ref(u.re.mk_empty(re_sort), m);
+        unsigned const n = p.num_ranges();
+        SASSERT(n > 0);
+        if (n == 1) {
+            auto [lo, hi] = p[0];
+            return mk_single_range_regex(u, lo, hi, re_sort);
+        }
+        // Build single-range AST nodes first, then sort by expression id
+        // so the resulting right-associated union matches the canonical
+        // id-sorted shape that seq_rewriter::merge_regex_sets expects.
+        // Without this the merge algorithm produces incorrect unions
+        // when it has to combine our materialized output with another
+        // (id-sorted) regex set.
+        expr_ref_vector ranges(m);
+        for (unsigned i = 0; i < n; ++i) {
+            auto [lo, hi] = p[i];
+            ranges.push_back(mk_single_range_regex(u, lo, hi, re_sort));
+        }
+        std::sort(ranges.data(), ranges.data() + ranges.size(),
+                  [](expr* a, expr* b) { return a->get_id() < b->get_id(); });
+        expr_ref acc(ranges.get(n - 1), m);
+        for (unsigned i = n - 1; i-- > 0; )
+            acc = expr_ref(u.re.mk_union(ranges.get(i), acc), m);
+        return acc;
+    }
+
+}

src/ast/rewriter/regex_range_collapse.h

@@ -0,0 +1,71 @@
+/*++
+Copyright (c) 2026 Microsoft Corporation
+
+Module Name:
+
+    regex_range_collapse.h
+
+Abstract:
+
+    Recognize regexes that are boolean combinations of character-class
+    primitives (re.empty, re.full_char, re.range with concrete chars,
+    and re.union/inter/comp/diff over translatable arguments), and
+    materialize a seq::range_predicate back into a canonical regex AST.
+
+    Together with seq_rewriter integration, this lets any boolean
+    combination of character-class regexes collapse to a canonical
+    multi-range form, so that equivalent character classes share AST
+    identity, and downstream consumers (derivative, OneStep, caching)
+    can short-circuit them as pure range predicates.
+
+Authors:
+
+    Margus Veanes (veanes) 2026
+
+--*/
+#pragma once
+
+#include "ast/rewriter/range_predicate.h"
+#include "ast/seq_decl_plugin.h"
+
+namespace seq {
+
+    /**
+     * If r is a boolean combination of character-class regex primitives
+     * over the unsigned character domain [0, max_char], compute the
+     * equivalent range_predicate and return true. Otherwise return false
+     * with out untouched.
+     *
+     * Recognized fragment (all character-class-preserving operations):
+     *   re.empty                              -> empty
+     *   re.full_char_set                      -> top
+     *   re.range "c_lo" "c_hi"  (concrete)    -> [c_lo, c_hi]
+     *   re.union r1 r2                        -> p1 | p2
+     *   re.intersection r1 r2                 -> p1 & p2
+     *   re.diff r1 r2                         -> p1 - p2
+     *
+     * Notably re.complement is NOT recognized: it is a SEQUENCE-level
+     * complement (over all of Σ*), not a character-class complement, so
+     * collapsing it would change semantics whenever the result is used
+     * in any non-character-class context.  Sequence-level rewrites for
+     * re.complement (double-comp, deMorgan, etc.) are handled directly
+     * in seq_rewriter::mk_re_complement.
+     */
+    bool regex_to_range_predicate(seq_util& u, expr* r, range_predicate& out);
+
+    /**
+     * Canonical materialization of p as a regex AST over the given
+     * sequence sort. Two range_predicates with equal canonical
+     * representations produce structurally identical regex ASTs:
+     *
+     *   empty                  -> re.empty
+     *   top                    -> re.full_char_set
+     *   single range [lo, hi]  -> re.range "lo" "hi"
+     *   multiple ranges        -> right-associated re.union of single
+     *                              ranges, in increasing order of lo
+     *                              (matching the canonical range order
+     *                              held by range_predicate).
+     */
+    expr_ref range_predicate_to_regex(seq_util& u, range_predicate const& p, sort* seq_sort);
+
+}

src/ast/rewriter/seq_derive.cpp

@@ -1252,21 +1252,18 @@ namespace seq {
     // Intersect the active suffix m_intervals[m_intervals_start..end] with [lo, hi]
     void derive::intersect_intervals(unsigned lo, unsigned hi) {
         // Copy active suffix to end, update start, then filter
-        unsigned old_start = m_intervals_start;
         unsigned old_sz = m_intervals.size();
-        for (unsigned i = old_start; i < old_sz; ++i)
+        for (unsigned i = m_intervals_start; i < old_sz; ++i)
             m_intervals.push_back(m_intervals[i]);
         m_intervals_start = old_sz;
-        // Filter in-place within new suffix
+        // Filter in-place within new suffix: drop intervals disjoint from [lo,hi],
+        // keep the intersection for overlapping ones.
         unsigned j = m_intervals_start;
         for (unsigned i = m_intervals_start; i < m_intervals.size(); ++i) {
             auto [lo1, hi1] = m_intervals[i];
-            if (hi < lo1 || lo > hi1) {
-                j = old_sz;
-                break;
-            }
-            if (hi1 >= lo)
-                m_intervals[j++] = {std::max(lo1, lo), std::min(hi1, hi)};
+            if (hi < lo1 || lo > hi1)
+                continue;  // disjoint with this interval — drop it
+            m_intervals[j++] = {std::max(lo1, lo), std::min(hi1, hi)};
         }
         m_intervals.shrink(j);
     }

src/ast/rewriter/seq_regex_bisim.cpp

@@ -102,9 +102,7 @@ namespace seq {
             expr* e = work.back();
             work.pop_back();
             expr* c = nullptr, * t = nullptr, * f = nullptr;
-            if (m.is_ite(e, c, t, f) ||
-                m_util.re.is_union(e, t, f) ||
-                m_util.re.is_antimirov_union(e, t, f)) {
+            if (m.is_ite(e, c, t, f)) {
                 if (seen.insert_if_not_there(t))
                     work.push_back(t);
                 if (seen.insert_if_not_there(f))

src/ast/rewriter/seq_rewriter.cpp

@@ -21,6 +21,7 @@ Authors:
 #include "util/uint_set.h"
 #include "ast/rewriter/seq_rewriter.h"
 #include "ast/rewriter/seq_regex_bisim.h"
+#include "ast/rewriter/regex_range_collapse.h"
 #include "ast/arith_decl_plugin.h"
 #include "ast/array_decl_plugin.h"
 #include "ast/ast_pp.h"
@@ -3961,6 +3962,32 @@ bool seq_rewriter::is_subset(expr* r1, expr* r2) const {
     return m_subset.is_subset(r1, r2);
 }
 
+bool seq_rewriter::try_collapse_re_union(expr* a, expr* b, expr_ref& result) {
+    sort* seq_sort = nullptr;
+    if (!u().is_re(a->get_sort(), seq_sort))
+        return false;
+    seq::range_predicate pa(u().max_char()), pb(u().max_char());
+    if (!seq::regex_to_range_predicate(u(), a, pa))
+        return false;
+    if (!seq::regex_to_range_predicate(u(), b, pb))
+        return false;
+    result = seq::range_predicate_to_regex(u(), pa | pb, seq_sort);
+    return true;
+}
+
+bool seq_rewriter::try_collapse_re_inter(expr* a, expr* b, expr_ref& result) {
+    sort* seq_sort = nullptr;
+    if (!u().is_re(a->get_sort(), seq_sort))
+        return false;
+    seq::range_predicate pa(u().max_char()), pb(u().max_char());
+    if (!seq::regex_to_range_predicate(u(), a, pa))
+        return false;
+    if (!seq::regex_to_range_predicate(u(), b, pb))
+        return false;
+    result = seq::range_predicate_to_regex(u(), pa & pb, seq_sort);
+    return true;
+}
+
 br_status seq_rewriter::mk_re_union0(expr* a, expr* b, expr_ref& result) {
     if (a == b) {
         result = a;
@@ -4005,11 +4032,15 @@ br_status seq_rewriter::mk_re_union0(expr* a, expr* b, expr_ref& result) {
         result = m().mk_ite(c, re().mk_union(a, r1), re().mk_union(a, r2));
         return BR_REWRITE3;
     }
+    if (try_collapse_re_union(a, b, result))
+        return BR_DONE;
     return BR_FAILED;
 }
 
 /* Creates a normalized union. */
 br_status seq_rewriter::mk_re_union(expr* a, expr* b, expr_ref& result) {
+    if (try_collapse_re_union(a, b, result))
+        return BR_DONE;
     result = mk_regex_union_normalize(a, b);
     return BR_DONE;
 }
@@ -4093,16 +4124,28 @@ br_status seq_rewriter::mk_re_inter0(expr* a, expr* b, expr_ref& result) {
         result = m().mk_ite(c, re().mk_inter(a, r1), re().mk_inter(a, r2));
         return BR_REWRITE3;
     }
+    if (try_collapse_re_inter(a, b, result))
+        return BR_DONE;
     return BR_FAILED;
 }
 
 /* Creates a normalized intersection. */
 br_status seq_rewriter::mk_re_inter(expr* a, expr* b, expr_ref& result) {
+    if (try_collapse_re_inter(a, b, result))
+        return BR_DONE;
     result = mk_regex_inter_normalize(a, b);
     return BR_DONE;
 }
 
 br_status seq_rewriter::mk_re_diff(expr* a, expr* b, expr_ref& result) {
+    seq::range_predicate pa(u().max_char()), pb(u().max_char());
+    sort* seq_sort = nullptr;
+    if (u().is_re(a->get_sort(), seq_sort)
+        && seq::regex_to_range_predicate(u(), a, pa)
+        && seq::regex_to_range_predicate(u(), b, pb)) {
+        result = seq::range_predicate_to_regex(u(), pa - pb, seq_sort);
+        return BR_DONE;
+    }
     result = mk_regex_inter_normalize(a, re().mk_complement(b));
     return BR_REWRITE2;
 }
@@ -5495,7 +5538,7 @@ void seq_rewriter::op_cache::cleanup() {
 lbool seq_rewriter::some_string_in_re(expr* r, zstring& s) {
     sort* rs;
     (void)rs;
-    // SASSERT(re().is_re(r, rs) && m_util.is_string(rs));
+    // SASSERT(u().is_re(r, rs) && m_util.is_string(rs));
     expr_mark visited;
     unsigned_vector str;
 

src/ast/rewriter/seq_rewriter.h

@@ -250,6 +250,14 @@ class seq_rewriter {
     br_status mk_re_union0(expr* a, expr* b, expr_ref& result);
     br_status mk_re_inter0(expr* a, expr* b, expr_ref& result);
     br_status mk_re_complement(expr* a, expr_ref& result);
+    // Range-set collapse helpers: if the operands form a boolean
+    // combination of character-class regexes, materialize the result as a
+    // canonical regex over a single range_predicate.  See
+    // ast/rewriter/regex_range_collapse.h for the recognized fragment.
+    // NOTE: re.complement is intentionally not in this set because it
+    // operates at the sequence level, not the character-class level.
+    bool try_collapse_re_union(expr* a, expr* b, expr_ref& result);
+    bool try_collapse_re_inter(expr* a, expr* b, expr_ref& result);
     br_status mk_re_star(expr* a, expr_ref& result);
     br_status mk_re_diff(expr* a, expr* b, expr_ref& result);
     br_status mk_re_xor(expr* a, expr* b, expr_ref& result);

src/test/CMakeLists.txt

@@ -125,6 +125,7 @@ add_executable(test-z3
   rational.cpp
   rcf.cpp
   region.cpp
+  regex_range_collapse.cpp
   sat_local_search.cpp
   sat_lookahead.cpp
   sat_user_scope.cpp

src/test/main.cpp

@@ -115,6 +115,7 @@
     X(arith_rewriter) \
     X(range_predicate) \
     X(range_predicate_translator) \
+    X(regex_range_collapse) \
     X(check_assumptions) \
     X(smt_context) \
     X(theory_dl) \

src/test/regex_range_collapse.cpp

@@ -0,0 +1,229 @@
+/*++
+Copyright (c) 2026 Microsoft Corporation
+
+Module Name:
+
+    regex_range_collapse.cpp - unit tests
+
+--*/
+
+#include "ast/rewriter/regex_range_collapse.h"
+#include "ast/reg_decl_plugins.h"
+#include "ast/ast_pp.h"
+#include "util/util.h"
+
+#include <iostream>
+
+namespace {
+
+    using seq::range_predicate;
+    using seq::regex_to_range_predicate;
+    using seq::range_predicate_to_regex;
+
+    static void check(bool ok, char const* what) {
+        if (!ok) {
+            std::cerr << "regex_range_collapse FAILED: " << what << "\n";
+            ENSURE(false);
+        }
+    }
+
+    static expr_ref mk_singleton_str(seq_util& u, unsigned c) {
+        return expr_ref(u.str.mk_string(zstring(c)), u.get_manager());
+    }
+
+    static bool extract_range_chars(seq_util& u, expr* e, unsigned& lo, unsigned& hi) {
+        expr* lo_e = nullptr; expr* hi_e = nullptr;
+        if (!u.re.is_range(e, lo_e, hi_e))
+            return false;
+        // Accept either string-constant or (seq.unit (Char N)) bound form.
+        if (u.re.is_range(e, lo, hi))
+            return true;
+        expr* lc = nullptr; expr* hc = nullptr;
+        if (u.str.is_unit(lo_e, lc) && u.is_const_char(lc, lo) &&
+            u.str.is_unit(hi_e, hc) && u.is_const_char(hc, hi))
+            return true;
+        return false;
+    }
+
+    static void run() {
+        ast_manager m;
+        reg_decl_plugins(m);
+        seq_util u(m);
+        unsigned const M = u.max_char();
+
+        sort* str_sort = u.str.mk_string_sort();
+        sort* re_sort  = u.re.mk_re(str_sort);
+
+        // primitives
+        {
+            range_predicate p(M);
+            check(regex_to_range_predicate(u, u.re.mk_empty(re_sort), p) && p.is_empty(),
+                  "re.empty -> empty");
+            check(regex_to_range_predicate(u, u.re.mk_full_char(re_sort), p) && p.is_top(),
+                  "re.full_char -> top");
+        }
+        // re.range "a" "z"
+        {
+            range_predicate p(M);
+            expr_ref a = mk_singleton_str(u, 'a');
+            expr_ref z = mk_singleton_str(u, 'z');
+            expr_ref r(u.re.mk_range(a, z), m);
+            check(regex_to_range_predicate(u, r, p) && p.num_ranges() == 1 &&
+                  p[0].first == 'a' && p[0].second == 'z',
+                  "re.range a z -> [a,z]");
+        }
+        // Disjoint union: (a..z) | (0..9)
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'z')), m);
+            expr_ref r2(u.re.mk_range(mk_singleton_str(u, '0'), mk_singleton_str(u, '9')), m);
+            expr_ref un(u.re.mk_union(r1, r2), m);
+            check(regex_to_range_predicate(u, un, p) && p.num_ranges() == 2,
+                  "(a-z)|(0-9) -> 2 ranges");
+            // canonical order: lower lo first
+            check(p[0].first == '0' && p[0].second == '9' && p[1].first == 'a' && p[1].second == 'z',
+                  "(a-z)|(0-9) ranges in canonical order");
+        }
+        // Overlapping union: (a..c) | (b..f) -> (a..f)
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'c')), m);
+            expr_ref r2(u.re.mk_range(mk_singleton_str(u, 'b'), mk_singleton_str(u, 'f')), m);
+            expr_ref un(u.re.mk_union(r1, r2), m);
+            check(regex_to_range_predicate(u, un, p) && p.num_ranges() == 1 &&
+                  p[0].first == 'a' && p[0].second == 'f',
+                  "(a-c)|(b-f) -> (a-f)");
+        }
+        // Adjacent union: (a..c) | (d..f) -> (a..f) (canonical predicate merges adjacent)
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'c')), m);
+            expr_ref r2(u.re.mk_range(mk_singleton_str(u, 'd'), mk_singleton_str(u, 'f')), m);
+            expr_ref un(u.re.mk_union(r1, r2), m);
+            check(regex_to_range_predicate(u, un, p) && p.num_ranges() == 1 &&
+                  p[0].first == 'a' && p[0].second == 'f',
+                  "(a-c)|(d-f) -> (a-f) via adjacency");
+        }
+        // Disjoint intersection: (a..z) & (0..9) -> empty
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'z')), m);
+            expr_ref r2(u.re.mk_range(mk_singleton_str(u, '0'), mk_singleton_str(u, '9')), m);
+            expr_ref ix(u.re.mk_inter(r1, r2), m);
+            check(regex_to_range_predicate(u, ix, p) && p.is_empty(),
+                  "(a-z)&(0-9) -> empty");
+        }
+        // Overlapping intersection: (a..f) & (c..z) -> (c..f)
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'f')), m);
+            expr_ref r2(u.re.mk_range(mk_singleton_str(u, 'c'), mk_singleton_str(u, 'z')), m);
+            expr_ref ix(u.re.mk_inter(r1, r2), m);
+            check(regex_to_range_predicate(u, ix, p) && p.num_ranges() == 1 &&
+                  p[0].first == 'c' && p[0].second == 'f',
+                  "(a-f)&(c-z) -> (c-f)");
+        }
+        // Complement: re.complement is intentionally NOT a char-class op
+        // (it operates over Σ*), so it must NOT be translated.
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'z')), m);
+            expr_ref cmp(u.re.mk_complement(r1), m);
+            check(!regex_to_range_predicate(u, cmp, p),
+                  "re.comp of range is NOT translatable (sequence-level complement)");
+        }
+        // Diff: (a..f) \ (c..z) -> (a..b)
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'f')), m);
+            expr_ref r2(u.re.mk_range(mk_singleton_str(u, 'c'), mk_singleton_str(u, 'z')), m);
+            expr_ref df(u.re.mk_diff(r1, r2), m);
+            check(regex_to_range_predicate(u, df, p) && p.num_ranges() == 1 &&
+                  p[0].first == 'a' && p[0].second == 'b',
+                  "(a-f) \\ (c-z) -> (a-b)");
+        }
+        // Negative: re.* of a range is NOT a char class
+        {
+            range_predicate p(M);
+            expr_ref r1(u.re.mk_range(mk_singleton_str(u, 'a'), mk_singleton_str(u, 'z')), m);
+            expr_ref star(u.re.mk_star(r1), m);
+            check(!regex_to_range_predicate(u, star, p),
+                  "re.* of range not translatable");
+        }
+
+        // ---- materialization round-trip ----
+
+        // empty -> re.empty
+        {
+            range_predicate p = range_predicate::empty(M);
+            expr_ref e = range_predicate_to_regex(u, p, str_sort);
+            check(u.re.is_empty(e), "empty -> re.empty");
+        }
+        // top -> re.full_char
+        {
+            range_predicate p = range_predicate::top(M);
+            expr_ref e = range_predicate_to_regex(u, p, str_sort);
+            check(u.re.is_full_char(e), "top -> re.full_char");
+        }
+        // single range -> re.range
+        {
+            range_predicate p = range_predicate::range('a', 'z', M);
+            expr_ref e = range_predicate_to_regex(u, p, str_sort);
+            unsigned lo = 0, hi = 0;
+            check(extract_range_chars(u, e, lo, hi) && lo == 'a' && hi == 'z',
+                  "[a-z] -> re.range a z");
+        }
+        // singleton -> re.range c c
+        {
+            range_predicate p = range_predicate::singleton('A', M);
+            expr_ref e = range_predicate_to_regex(u, p, str_sort);
+            unsigned lo = 0, hi = 0;
+            check(extract_range_chars(u, e, lo, hi) && lo == 'A' && hi == 'A',
+                  "{A} -> re.range A A");
+        }
+        // 2 ranges -> re.union(range_0, range_1) in canonical order
+        {
+            range_predicate p = range_predicate::range('0', '9', M)
+                              | range_predicate::range('a', 'z', M);
+            expr_ref e = range_predicate_to_regex(u, p, str_sort);
+            expr* a = nullptr; expr* b = nullptr;
+            check(u.re.is_union(e, a, b), "2-range -> union");
+            unsigned lo0 = 0, hi0 = 0, lo1 = 0, hi1 = 0;
+            check(extract_range_chars(u, a, lo0, hi0) && lo0 == '0' && hi0 == '9',
+                  "union arg0 = (0-9) (canonical: lower lo first)");
+            check(extract_range_chars(u, b, lo1, hi1) && lo1 == 'a' && hi1 == 'z',
+                  "union arg1 = (a-z)");
+        }
+        // 3 ranges -> right-associated union
+        {
+            range_predicate p = range_predicate::range(0, 5, M)
+                              | range_predicate::range(10, 15, M)
+                              | range_predicate::range(20, 25, M);
+            expr_ref e = range_predicate_to_regex(u, p, str_sort);
+            expr* a = nullptr; expr* rest = nullptr;
+            check(u.re.is_union(e, a, rest), "3-range -> union(...)");
+            unsigned lo = 0, hi = 0;
+            check(extract_range_chars(u, a, lo, hi) && lo == 0 && hi == 5, "first arg = (0-5)");
+            expr* b = nullptr; expr* c = nullptr;
+            check(u.re.is_union(rest, b, c), "rest is union(...,...)");
+            check(extract_range_chars(u, b, lo, hi) && lo == 10 && hi == 15, "second range");
+            check(extract_range_chars(u, c, lo, hi) && lo == 20 && hi == 25, "third range");
+        }
+        // Round-trip identity for an arbitrary range-set
+        {
+            range_predicate p_in = range_predicate::range('a', 'c', M)
+                                 | range_predicate::range('m', 'p', M)
+                                 | range_predicate::range('x', 'z', M);
+            expr_ref e = range_predicate_to_regex(u, p_in, str_sort);
+            range_predicate p_out(M);
+            check(regex_to_range_predicate(u, e, p_out), "round-trip translatable");
+            check(p_in == p_out, "round-trip equal");
+        }
+
+        std::cerr << "regex_range_collapse tests passed\n";
+    }
+}
+
+void tst_regex_range_collapse() {
+    run();
+}