diff --git a/builtin/commands/default.lua b/builtin/commands/default.lua
index cb71f68..af31814 100644
--- a/builtin/commands/default.lua
+++ b/builtin/commands/default.lua
@@ -122,15 +122,23 @@ cmd { name = "buffer.self-insert",
 -- C-s / C-r begin a live in-buffer isearch: the match under the cursor
 -- highlights as you type, the same key steps to the next/previous
 -- match, RET accepts (keeping the highlights until the next edit), and
--- C-g / Esc restore the pre-search cursor. While a search is running
--- every keystroke is intercepted in Rust (dispatch_search_key), so
--- these commands only run to *start* a search from an idle keymap.
+-- C-g / Esc restore the pre-search cursor. C-M-s / C-M-r start a regex
+-- search, and M-r toggles literal <-> regex mid-search (handled in
+-- Rust). While a search is running every keystroke is intercepted in
+-- Rust (dispatch_search_key), so these commands only run to *start* a
+-- search from an idle keymap. ed.search_start(forward, regex).
 cmd { name = "search.forward",
       description = "Start an incremental search forward from the cursor.",
-      fn = function() ed.search_start(true) end }
+      fn = function() ed.search_start(true, false) end }
 cmd { name = "search.backward",
       description = "Start an incremental search backward from the cursor.",
-      fn = function() ed.search_start(false) end }
+      fn = function() ed.search_start(false, false) end }
+cmd { name = "search.forward-regex",
+      description = "Start an incremental regex search forward from the cursor.",
+      fn = function() ed.search_start(true, true) end }
+cmd { name = "search.backward-regex",
+      description = "Start an incremental regex search backward from the cursor.",
+      fn = function() ed.search_start(false, true) end }
 
 -- History --------------------------------------------------------------------
 
diff --git a/builtin/keymaps/default.lua b/builtin/keymaps/default.lua
index 46fe6c4..18b9386 100644
--- a/builtin/keymaps/default.lua
+++ b/builtin/keymaps/default.lua
@@ -59,8 +59,12 @@ bind("TAB", "buffer.tab")
 -- adds isearch without colliding with the CUA / Emacs editing keys.
 -- Once a search is running, C-s / C-r step to the next / previous
 -- match; that interception happens in Rust, so it needs no binding.
+-- C-M-s / C-M-r start a regex search (Emacs isearch-forward-regexp);
+-- M-r toggles literal <-> regex mid-search (intercepted in Rust).
 bind("C-s", "search.forward")
 bind("C-r", "search.backward")
+bind("C-M-s", "search.forward-regex")
+bind("C-M-r", "search.backward-regex")
 
 -- CUA-style word-level deletion (the same shortcuts users expect from
 -- IDEs, browsers, terminals on Linux/Windows). C-BS deletes back to
diff --git a/src/editor.rs b/src/editor.rs
index 42be2cb..174db2e 100644
--- a/src/editor.rs
+++ b/src/editor.rs
@@ -641,6 +641,7 @@ impl EditorState {
     /// * `RET`                    --- accept (keep cursor + highlights).
     /// * `C-g` / `Esc`            --- cancel (restore origin cursor).
     /// * `BS`                     --- shorten the query by one char.
+    /// * `M-r`                    --- toggle literal ↔ regex (Q#RX3).
     /// * a printable char         --- extend the query.
     ///
     /// Unrecognized chords are swallowed (an active isearch eats every
@@ -654,6 +655,7 @@ impl EditorState {
             SearchKey::Accept => self.core.borrow_mut().search_finish(true),
             SearchKey::Cancel => self.core.borrow_mut().search_finish(false),
             SearchKey::Backspace => self.core.borrow_mut().search_backspace(),
+            SearchKey::ToggleRegex => self.core.borrow_mut().search_toggle_regex(),
             SearchKey::Insert(ch) => self.core.borrow_mut().search_input_char(ch),
             SearchKey::Ignore => {}
         }
@@ -1221,6 +1223,8 @@ enum SearchKey {
     Cancel,
     /// Shorten the query by one character (BS).
     Backspace,
+    /// Toggle literal ↔ regex matching (M-r; Q#RX3).
+    ToggleRegex,
     /// Extend the query with a printable character.
     Insert(char),
     /// Unhandled --- swallowed without complaint.
@@ -1260,6 +1264,14 @@ impl SearchKey {
                 _ => Self::Ignore,
             };
         }
+        // M-r toggles regex mode (Q#RX3). Alt-only chord, distinct from
+        // the C-r (previous-match) above.
+        if alt
+            && !ctrl
+            && let KeyCode::Char('r') = chord.code
+        {
+            return Self::ToggleRegex;
+        }
         Self::Ignore
     }
 }
@@ -1726,24 +1738,28 @@ fn paint_minibuffer(
 
 /// Paint the incremental-search prompt on the bottom row:
 /// `I-search: <query>  (n/m)`. Backward searches read `I-search
-/// backward:`; a non-empty query with no matches reads `[no match]`.
-/// Overwrites the status line painted just before it. The terminal
-/// cursor is *not* returned here — it stays in the buffer at the
-/// active match (see [`paint_frame`]).
+/// backward:`; regex searches prefix `Regex `; a non-empty query with
+/// no matches reads `[no match]`, and an uncompilable regex reads
+/// `[invalid]`. Overwrites the status line painted just before it. The
+/// terminal cursor is *not* returned here — it stays in the buffer at
+/// the active match (see [`paint_frame`]).
 fn paint_search_prompt(
     grid: &mut crate::cell::CellGrid<'_>,
     core: &EditorCore,
     term_size: crate::cell::CellSize,
 ) {
-    let prompt = if core.search_forward() {
-        "I-search: "
-    } else {
-        "I-search backward: "
+    let prompt = match (core.search_is_regex(), core.search_forward()) {
+        (false, true) => "I-search: ",
+        (false, false) => "I-search backward: ",
+        (true, true) => "Regex I-search: ",
+        (true, false) => "Regex I-search backward: ",
     };
     let query = core.search_query();
     let (active, total) = core.search_match_summary();
     let suffix = if query.is_empty() {
         String::new()
+    } else if core.search_is_invalid() {
+        "  [invalid]".to_string()
     } else if total == 0 {
         "  [no match]".to_string()
     } else {
@@ -2106,6 +2122,40 @@ mod tests {
         assert_eq!(s.core.borrow().search_query(), "foo");
     }
 
+    #[test]
+    fn regex_isearch_via_dispatch_c_m_s() {
+        let mut s = fresh_with(b"a1 b2 c3");
+        s.core.borrow_mut().active_window_mut().cursor = 0;
+        // C-M-s starts a regex search (search.forward-regex).
+        s.dispatch_key(
+            FrontendId::LOCAL,
+            key(
+                KeyCode::Char('s'),
+                KeyModifiers::CONTROL | KeyModifiers::ALT,
+            ),
+        );
+        assert!(s.core.borrow().search_active());
+        assert!(s.core.borrow().search_is_regex());
+        type_chars(&mut s, r"\d");
+        assert_eq!(s.core.borrow().search_match_summary().1, 3);
+    }
+
+    #[test]
+    fn m_r_toggles_regex_mid_search() {
+        let mut s = fresh_with(b"a.b axb");
+        s.core.borrow_mut().active_window_mut().cursor = 0;
+        s.dispatch_key(FrontendId::LOCAL, ctrl('s')); // literal
+        type_chars(&mut s, "a.b");
+        assert_eq!(s.core.borrow().search_match_summary().1, 1);
+        // M-r toggles to regex (intercepted in dispatch_search_key).
+        s.dispatch_key(
+            FrontendId::LOCAL,
+            key(KeyCode::Char('r'), KeyModifiers::ALT),
+        );
+        assert!(s.core.borrow().search_is_regex());
+        assert_eq!(s.core.borrow().search_match_summary().1, 2);
+    }
+
     #[test]
     fn isearch_accumulates_across_renders_like_run_loop() {
         // Reproduce the real run loop: a render between every keystroke
diff --git a/src/editor_core.rs b/src/editor_core.rs
index 9c0feb2..c289158 100644
--- a/src/editor_core.rs
+++ b/src/editor_core.rs
@@ -78,6 +78,14 @@ pub struct SearchSession {
     /// Drives the prompt label ("I-search" vs "I-search backward")
     /// and the wrap direction of an empty-query repeat.
     forward: bool,
+    /// Whether the query is a regex (Q#RX3). `false` = smart-case
+    /// substring (`find_all`); `true` = smart-case regex
+    /// (`find_all_regex`). Toggled live by `M-r`.
+    regex: bool,
+    /// `true` when the last recompute's regex pattern failed to compile
+    /// — the prompt shows `[invalid]` instead of a match count. Always
+    /// `false` in literal mode (substring never "fails to compile").
+    invalid: bool,
 }
 
 /// The world state mutated by editor commands.
@@ -536,6 +544,21 @@ impl EditorCore {
         self.search.as_ref().is_none_or(|s| s.forward)
     }
 
+    /// `true` iff the active search is in regex mode (Q#RX3). `false`
+    /// for literal substring, or when no search is running.
+    #[must_use]
+    pub fn search_is_regex(&self) -> bool {
+        self.search.as_ref().is_some_and(|s| s.regex)
+    }
+
+    /// `true` iff the active regex search's pattern failed to compile —
+    /// the prompt shows `[invalid]` rather than a match count. Always
+    /// `false` in literal mode / when no search is running.
+    #[must_use]
+    pub fn search_is_invalid(&self) -> bool {
+        self.search.as_ref().is_some_and(|s| s.invalid)
+    }
+
     /// `(active_index, total)` for the active buffer's matches, for the
     /// prompt's "n/m" readout. `active_index` is 0-based and `None`
     /// when there are no matches.
@@ -552,11 +575,12 @@ impl EditorCore {
     }
 
     /// Begin an incremental search anchored at the active buffer +
-    /// cursor. `forward` sets the initial step direction. A no-op if a
-    /// search is already running (the entry chord is intercepted while
-    /// active, so this is only reached from an inactive state — the
-    /// guard is belt-and-suspenders).
-    pub fn search_begin(&mut self, forward: bool) {
+    /// cursor. `forward` sets the initial step direction; `regex`
+    /// selects regex (`true`) vs literal substring (`false`) matching.
+    /// A no-op if a search is already running (the entry chord is
+    /// intercepted while active, so this is only reached from an
+    /// inactive state — the guard is belt-and-suspenders).
+    pub fn search_begin(&mut self, forward: bool, regex: bool) {
         if self.search.is_some() {
             return;
         }
@@ -571,9 +595,22 @@ impl EditorCore {
             query: String::new(),
             origin,
             forward,
+            regex,
+            invalid: false,
         });
     }
 
+    /// Toggle the active search between literal and regex matching
+    /// (Q#RX3, `M-r`), re-running the current query in the new mode. A
+    /// no-op when no search is running.
+    pub fn search_toggle_regex(&mut self) {
+        let Some(session) = self.search.as_mut() else {
+            return;
+        };
+        session.regex = !session.regex;
+        self.search_recompute();
+    }
+
     /// Ensure the active window carries a [`crate::search::SearchView`]
     /// overlay, attaching one if absent (deduped by overlay kind). The
     /// view reads the per-buffer [`Self::search_store`] keyed on the
@@ -617,8 +654,23 @@ impl EditorCore {
         let bid = session.origin.0;
         let origin_byte = session.origin.1;
         let query = session.query.clone();
+        let regex = session.regex;
         let bytes = self.buffer_bytes(bid);
-        let matches = crate::search::find_all(&bytes, &query);
+        // Regex: `None` ⇒ the pattern won't compile (mark invalid, drop
+        // matches). Literal substring never fails. An invalid pattern
+        // clears the store (no stale matches paint) and shows
+        // `[invalid]` via the prompt.
+        let (matches, invalid) = if regex {
+            match crate::search::find_all_regex(&bytes, &query) {
+                Some(m) => (m, false),
+                None => (Vec::new(), true),
+            }
+        } else {
+            (crate::search::find_all(&bytes, &query), false)
+        };
+        if let Some(session) = self.search.as_mut() {
+            session.invalid = invalid;
+        }
         let focus = {
             let mut guard = self
                 .search_store
@@ -2422,7 +2474,7 @@ mod tests {
         let mut s = from_bytes(b"foo bar foo baz foo");
         let bid = s.active_buffer_id();
         s.active_window_mut().cursor = 0;
-        s.search_begin(true);
+        s.search_begin(true, false);
         assert!(s.search_active());
         type_query(&mut s, "foo");
         // Three matches: 0..3, 8..11, 16..19; first (at/after origin 0)
@@ -2438,7 +2490,7 @@ mod tests {
     fn search_step_walks_matches_and_wraps() {
         let mut s = from_bytes(b"foo bar foo baz foo");
         s.active_window_mut().cursor = 0;
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "foo");
         assert_eq!(s.cursor(), 0);
         s.search_step(true);
@@ -2455,7 +2507,7 @@ mod tests {
     fn search_focuses_first_match_at_or_after_origin() {
         let mut s = from_bytes(b"foo bar foo");
         s.active_window_mut().cursor = 5; // inside "bar"
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "foo");
         // First match with start >= 5 is the one at byte 8.
         assert_eq!(s.cursor(), 8);
@@ -2467,7 +2519,7 @@ mod tests {
         let mut s = from_bytes(b"foo bar foo");
         let bid = s.active_buffer_id();
         s.active_window_mut().cursor = 5;
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "foo");
         assert_eq!(s.cursor(), 8);
         s.search_finish(false); // cancel
@@ -2488,7 +2540,7 @@ mod tests {
         let mut s = from_bytes(b"foo bar foo");
         let bid = s.active_buffer_id();
         s.active_window_mut().cursor = 0;
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "foo");
         s.search_step(true); // focus the match at byte 8
         assert_eq!(s.cursor(), 8);
@@ -2509,7 +2561,7 @@ mod tests {
     fn search_backspace_widens_the_match_set() {
         let mut s = from_bytes(b"fo foo food");
         s.active_window_mut().cursor = 0;
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "foo"); // matches "foo" at 3..6, 7..10
         assert_eq!(s.search_match_summary().1, 2);
         s.search_backspace(); // query "fo"
@@ -2521,7 +2573,7 @@ mod tests {
     fn search_smart_case_is_case_sensitive_with_uppercase() {
         let mut s = from_bytes(b"Foo foo FOO");
         s.active_window_mut().cursor = 0;
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "Foo"); // uppercase => case-sensitive
         assert_eq!(s.search_match_summary().1, 1);
         s.search_backspace();
@@ -2536,7 +2588,7 @@ mod tests {
         let mut s = from_bytes(b"foo foo");
         let bid = s.active_buffer_id();
         s.active_window_mut().cursor = 0;
-        s.search_begin(true);
+        s.search_begin(true, false);
         type_query(&mut s, "foo");
         s.search_finish(true); // matches persist after accept
         assert!(!s.search_store.lock().expect("store").is_stale(bid));
@@ -2546,4 +2598,50 @@ mod tests {
             "an edit marks the buffer's matches stale (linger fix)"
         );
     }
+
+    // ---- regex search (Q#RX3) ------------------------------------------
+
+    #[test]
+    fn regex_search_matches_pattern() {
+        let mut s = from_bytes(b"a1 b2 c3");
+        s.active_window_mut().cursor = 0;
+        s.search_begin(true, true);
+        assert!(s.search_is_regex());
+        type_query(&mut s, r"\d");
+        assert_eq!(s.search_match_summary().1, 3, "\\d matches 1, 2, 3");
+        assert!(!s.search_is_invalid());
+    }
+
+    #[test]
+    fn regex_invalid_pattern_flags_and_recovers() {
+        let mut s = from_bytes(b"foo");
+        s.active_window_mut().cursor = 0;
+        s.search_begin(true, true);
+        type_query(&mut s, "fo("); // unbalanced group mid-typing
+        assert!(s.search_is_invalid(), "incomplete group is invalid");
+        assert_eq!(s.search_match_summary().1, 0, "invalid ⇒ no matches");
+        type_query(&mut s, "o)"); // completes the group: regex fo(o) → "foo"
+        assert!(!s.search_is_invalid(), "valid pattern recovers");
+        assert_eq!(s.search_match_summary().1, 1);
+    }
+
+    #[test]
+    fn toggle_regex_reinterprets_the_query() {
+        let mut s = from_bytes(b"a.b axb");
+        s.active_window_mut().cursor = 0;
+        s.search_begin(true, false); // literal
+        type_query(&mut s, "a.b");
+        assert!(!s.search_is_regex());
+        assert_eq!(
+            s.search_match_summary().1,
+            1,
+            "literal '.' matches only a.b"
+        );
+        s.search_toggle_regex(); // → regex
+        assert!(s.search_is_regex());
+        assert_eq!(s.search_match_summary().1, 2, "regex '.' also matches axb");
+        s.search_toggle_regex(); // back to literal
+        assert!(!s.search_is_regex());
+        assert_eq!(s.search_match_summary().1, 1);
+    }
 }
diff --git a/src/lua_bindings.rs b/src/lua_bindings.rs
index a834a23..7cee70e 100644
--- a/src/lua_bindings.rs
+++ b/src/lua_bindings.rs
@@ -11439,13 +11439,14 @@ fn install_history(editor: &Table, lua: &Lua, core: &SharedCore) -> mlua::Result
 /// post-accept navigation commands can begin / step a search from Lua.
 fn install_search(editor: &Table, lua: &Lua, core: &SharedCore) -> mlua::Result<()> {
     {
-        // search_start(forward): begin an isearch in the given
-        // direction, anchored at the active buffer + cursor.
+        // search_start(forward, regex): begin an isearch in the given
+        // direction, anchored at the active buffer + cursor. `regex`
+        // selects regex vs literal substring matching (Q#RX3).
         let cc = core.clone();
         editor.set(
             "search_start",
-            lua.create_function(move |_, forward: bool| {
-                cc.borrow_mut().search_begin(forward);
+            lua.create_function(move |_, (forward, regex): (bool, bool)| {
+                cc.borrow_mut().search_begin(forward, regex);
                 Ok(())
             })?,
         )?;
diff --git a/src/search.rs b/src/search.rs
index 5e974bc..2fbffa7 100644
--- a/src/search.rs
+++ b/src/search.rs
@@ -363,45 +363,65 @@ impl View for SearchView {
         let cell_origin = viewport.cell_origin;
 
         for m in &matches {
-            let line = crate::diag::line_at_offset(&line_offsets, m.start as u32);
-            if line < start_line_buf {
-                continue;
-            }
-            let row_offset = line - start_line_buf;
-            if row_offset >= max_rows {
-                break;
-            }
-            let line_start = line_offsets[line as usize];
-            let line_end = line_offsets
-                .get(line as usize + 1)
-                .copied()
-                .unwrap_or(source.len() as u32);
-            let line_end_no_nl = if line_end > line_start
-                && source.get(line_end as usize - 1).copied() == Some(b'\n')
-            {
-                line_end - 1
-            } else {
-                line_end
-            };
-            let line_bytes = &source[line_start as usize..line_end_no_nl as usize];
-            let within_start = (m.start as u32).saturating_sub(line_start) as usize;
-            let within_end = (m.end as u32).saturating_sub(line_start) as usize;
-            let (start_col, end_col) =
-                crate::diag::byte_range_to_display_cols(line_bytes, within_start, within_end);
-            if end_col <= start_col {
-                continue;
-            }
             let style = if Some(*m) == active {
                 active_match_style()
             } else {
                 match_style()
             };
-            let cell_row = cell_origin.row + row_offset;
-            let clamped_start = start_col.min(max_cols);
-            let clamped_end = end_col.min(max_cols);
-            for col in clamped_start..clamped_end {
-                let cell = cells.at(CellCoord::new(cell_row, cell_origin.col + col));
-                cell.style = merge_styles(cell.style, style);
+            // A regex match may span multiple lines (Q#RX4); wash each
+            // row's clipped slice, mirroring the selection renderer.
+            // Single-line matches (every literal match) touch one row.
+            let first_line = crate::diag::line_at_offset(&line_offsets, m.start as u32);
+            // Matches are sorted ascending, so once one starts below the
+            // viewport every later one does too — stop.
+            if first_line >= start_line_buf.saturating_add(max_rows) {
+                break;
+            }
+            let last_byte = m.end.saturating_sub(1).max(m.start) as u32;
+            let last_line = crate::diag::line_at_offset(&line_offsets, last_byte);
+            for line in first_line..=last_line {
+                if line < start_line_buf {
+                    continue;
+                }
+                let row_offset = line - start_line_buf;
+                if row_offset >= max_rows {
+                    break;
+                }
+                let line_start = line_offsets[line as usize];
+                let line_end = line_offsets
+                    .get(line as usize + 1)
+                    .copied()
+                    .unwrap_or(source.len() as u32);
+                let line_end_no_nl = if line_end > line_start
+                    && source.get(line_end as usize - 1).copied() == Some(b'\n')
+                {
+                    line_end - 1
+                } else {
+                    line_end
+                };
+                // Clip the match to this line's content (newline excluded
+                // so a multi-line match doesn't wash a phantom trailing
+                // cell).
+                let paint_start = (m.start as u32).max(line_start);
+                let paint_end = (m.end as u32).min(line_end_no_nl);
+                if paint_start >= paint_end {
+                    continue;
+                }
+                let line_bytes = &source[line_start as usize..line_end_no_nl as usize];
+                let within_start = (paint_start - line_start) as usize;
+                let within_end = (paint_end - line_start) as usize;
+                let (start_col, end_col) =
+                    crate::diag::byte_range_to_display_cols(line_bytes, within_start, within_end);
+                if end_col <= start_col {
+                    continue;
+                }
+                let cell_row = cell_origin.row + row_offset;
+                let clamped_start = start_col.min(max_cols);
+                let clamped_end = end_col.min(max_cols);
+                for col in clamped_start..clamped_end {
+                    let cell = cells.at(CellCoord::new(cell_row, cell_origin.col + col));
+                    cell.style = merge_styles(cell.style, style);
+                }
             }
         }
     }
@@ -607,6 +627,66 @@ mod tests {
         );
     }
 
+    #[test]
+    fn search_view_washes_a_multiline_match_per_row() {
+        use crate::cell::{Cell, CellGrid, CellSize};
+        use crate::view::Viewport;
+
+        let store = make_shared_store();
+        let bid = BufferId::next();
+        let mut buf = Buffer::new(bid, "t.txt");
+        // "foo\nbar\nbaz": match [0,7) = "foo\nbar" spans lines 0–1.
+        buf.apply_edit(crate::buffer::EditOp::Insert {
+            pos: 0,
+            bytes: b"foo\nbar\nbaz",
+        })
+        .expect("seed");
+        store.lock().unwrap().set(bid, r"foo\nbar", vec![r(0, 7)]);
+
+        let (rows, cols) = (3u32, 10u32);
+        let mut backing = vec![Cell::default(); (rows * cols) as usize];
+        let mut grid = CellGrid {
+            cells: &mut backing,
+            stride: cols,
+            size: CellSize::new(rows, cols),
+        };
+        SearchView::new(store.clone()).render(
+            &buf,
+            Viewport {
+                buffer_start: 0,
+                buffer_end: buf.len(),
+                cell_origin: CellCoord::new(0, 0),
+                cell_size: CellSize::new(rows, cols),
+            },
+            &mut grid,
+        );
+
+        // Row 0 "foo" and row 1 "bar" both wash (the active match's
+        // bright color); the newline cells and row 2 "baz" do not.
+        for col in 0..3 {
+            assert_eq!(
+                grid.get(CellCoord::new(0, col)).style.bg,
+                Color::Indexed(11),
+                "row 0 col {col} should wash"
+            );
+            assert_eq!(
+                grid.get(CellCoord::new(1, col)).style.bg,
+                Color::Indexed(11),
+                "row 1 col {col} should wash"
+            );
+        }
+        assert_eq!(
+            grid.get(CellCoord::new(0, 3)).style.bg,
+            Color::Default,
+            "the newline cell past 'foo' is not washed"
+        );
+        assert_eq!(
+            grid.get(CellCoord::new(2, 0)).style.bg,
+            Color::Default,
+            "row 2 'baz' is outside the match"
+        );
+    }
+
     #[test]
     fn store_staleness_tracks_per_buffer() {
         let mut s = SearchStore::new();