Regex: don't overflow the stack on long lines

libstdc++'s std::regex matcher recurses about once per matched
character, so '.*', 'a+' or '(a|b)*' on a line of a few tens of
thousands of characters (minified JS/JSON) overflowed the 8 MiB stack
and crashed the editor from the regex search prompt or regex replace.

Regex search and regex replace now run their matching on a worker
thread with a 1 GiB reserved stack (RunWithLargeStack; the caller
waits, so nothing is shared concurrently). Renderers skip regex match
highlighting on lines over 10,000 bytes. Catastrophic backtracking
(e.g. (a*)*b) is still possible: std::regex has no time limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

CMakeLists.txt

@@ -172,6 +172,7 @@ set(COMMON_SOURCES
         ErrorHandler.cc
         SyscallWrappers.cc
         ErrorRecovery.cc
+        RegexGuard.cc
         TerminalInputHandler.cc
         TerminalRenderer.cc
         TerminalFrontend.cc
@@ -384,6 +385,7 @@ if (BUILD_TESTS)
             ErrorHandler.cc
             SyscallWrappers.cc
             ErrorRecovery.cc
+            RegexGuard.cc
             KKeymap.cc
             SwapRecorder.h
             OptimizedSearch.cc

Command.cc

@@ -11,6 +11,7 @@
 #include <string_view>
 
 #include "Command.h"
+#include "RegexGuard.h"
 #include "syntax/HighlighterRegistry.h"
 #include "syntax/NullHighlighter.h"
 #include "Editor.h"
@@ -797,6 +798,8 @@ search_compute_matches_regex(const Buffer &buf, const std::string &pattern, std:
 	try {
 		const std::regex rx(pattern);
 		const auto &rows = buf.Rows();
+		// std::regex recurses per matched character; see RegexGuard.h.
+		kte::RunWithLargeStack([&] {
 			for (std::size_t y = 0; y < rows.size(); ++y) {
 				std::string line = static_cast<std::string>(rows[y]);
 				for (auto it = std::sregex_iterator(line.begin(), line.end(), rx);
@@ -807,6 +810,7 @@ search_compute_matches_regex(const Buffer &buf, const std::string &pattern, std:
 					});
 				}
 			}
+		});
 	} catch (const std::regex_error &e) {
 		err_out = e.what();
 		// Return empty results on error
@@ -3212,6 +3216,9 @@ cmd_newline(CommandContext &ctx)
 			std::size_t nrows = buf->Nrows();
 			if (nrows > 1 && buf->GetLineString(nrows - 1).empty())
 				--nrows;
+			// std::regex recurses per matched character; see RegexGuard.h. The
+			// caller waits for the worker, so the buffer is never shared.
+			kte::RunWithLargeStack([&] {
 				for (std::size_t y = 0; y < nrows; ++y) {
 					const std::string before = buf->GetLineString(y);
 					const std::string after  = std::regex_replace(before, rx, repl);
@@ -3224,6 +3231,7 @@ cmd_newline(CommandContext &ctx)
 						++changed;
 					}
 				}
+			});
 			clamp_cursor_to_buffer(*buf);
 			buf->SetDirty(true);
 			ctx.editor.SetStatus("Regex replaced in " + std::to_string(changed) + " line(s)");

ImGuiRenderer.cc

@@ -10,6 +10,7 @@
 #include <regex>
 
 #include "ImGuiRenderer.h"
+#include "RegexGuard.h"
 #include "Highlight.h"
 #include "GUITheme.h"
 #include "Buffer.h"
@@ -359,7 +360,9 @@ ImGuiRenderer::Draw(Editor &ed)
 			if (search_mode) {
 				// In regex mode, reuse the compiled regex hoisted above the loop.
 				if (regex_mode) {
-					if (search_rx_valid) {
+					// Long lines are not highlighted: std::regex recursion
+					// could overflow the stack (RegexGuard.h).
+					if (search_rx_valid && line.size() <= kte::kRegexRenderLineLimit) {
 						try {
 							for (auto it = std::sregex_iterator(line.begin(), line.end(), search_rx);
 							     it != std::sregex_iterator(); ++it) {

QtFrontend.cc

@@ -1,4 +1,5 @@
 #include "QtFrontend.h"
+#include "RegexGuard.h"
 
 #include <QApplication>
 #include <QWidget>
@@ -165,7 +166,9 @@ protected:
 						    (ed_->CurrentPromptKind() == Editor::PromptKind::RegexSearch ||
 						     ed_->CurrentPromptKind() ==
 						     Editor::PromptKind::RegexReplaceFind)) {
-							try {
+							// Long lines are not highlighted: std::regex recursion
+							// could overflow the stack (RegexGuard.h).
+							if (line.size() <= kte::kRegexRenderLineLimit) try {
 								std::regex rx(ed_->SearchQuery());
 								for (auto it = std::sregex_iterator(
 									     line.begin(), line.end(), rx);

RegexGuard.cc

@@ -0,0 +1,51 @@
+#include "RegexGuard.h"
+
+#include <exception>
+#include <pthread.h>
+
+namespace kte {
+namespace {
+// 1 GiB supports matches spanning several million characters.
+constexpr std::size_t kLargeStackBytes = std::size_t{1} << 30;
+
+struct Job {
+	const std::function<void()> *fn;
+	std::exception_ptr error;
+};
+
+
+void *
+run_job(void *arg)
+{
+	auto *job = static_cast<Job *>(arg);
+	try {
+		(*job->fn)();
+	} catch (...) {
+		job->error = std::current_exception();
+	}
+	return nullptr;
+}
+} // namespace
+
+
+void
+RunWithLargeStack(const std::function<void()> &fn)
+{
+	Job job{&fn, nullptr};
+	pthread_attr_t attr;
+	bool started = false;
+	pthread_t tid{};
+	if (pthread_attr_init(&attr) == 0) {
+		if (pthread_attr_setstacksize(&attr, kLargeStackBytes) == 0)
+			started = pthread_create(&tid, &attr, run_job, &job) == 0;
+		pthread_attr_destroy(&attr);
+	}
+	if (!started) {
+		fn();
+		return;
+	}
+	pthread_join(tid, nullptr);
+	if (job.error)
+		std::rethrow_exception(job.error);
+}
+} // namespace kte

RegexGuard.h

@@ -0,0 +1,22 @@
+// RegexGuard.h - keep std::regex from overflowing the stack
+//
+// libstdc++'s std::regex matcher recurses roughly once per character of a
+// match, so a pattern such as ".*" or "a+" on a line of a few tens of
+// thousands of characters (minified JS/JSON) overflows a default 8 MiB
+// stack and crashes the editor.
+#pragma once
+
+#include <cstddef>
+#include <functional>
+
+namespace kte {
+// Renderers highlight regex matches only on lines up to this many bytes;
+// longer lines are drawn without match highlighting.
+constexpr std::size_t kRegexRenderLineLimit = 10000;
+
+// Run fn to completion on a thread with a large stack (address space is
+// reserved, pages are committed only as used), so regex work over long lines
+// cannot overflow. If such a thread cannot be created, fn runs on the calling
+// thread. Exceptions thrown by fn are rethrown to the caller.
+void RunWithLargeStack(const std::function<void()> &fn);
+} // namespace kte

TerminalRenderer.cc

@@ -10,6 +10,7 @@
 #include <string>
 
 #include "TerminalRenderer.h"
+#include "RegexGuard.h"
 #include "Buffer.h"
 #include "Editor.h"
 #include "Highlight.h"
@@ -70,7 +71,9 @@ TerminalRenderer::Draw(Editor &ed)
 				if (ed.PromptActive() && (
 					    ed.CurrentPromptKind() == Editor::PromptKind::RegexSearch || ed.
 					    CurrentPromptKind() == Editor::PromptKind::RegexReplaceFind)) {
-					try {
+					// Long lines are not highlighted: std::regex recursion
+					// could overflow the stack (RegexGuard.h).
+					if (sline.size() <= kte::kRegexRenderLineLimit) try {
 						std::regex rx(ed.SearchQuery());
 						for (auto it = std::sregex_iterator(sline.begin(), sline.end(), rx);
 						     it != std::sregex_iterator(); ++it) {

tests/test_audit_regressions.cc

@@ -695,3 +695,24 @@ TEST(Audit_UArg_DoesNotOverflow)
 		ed.UArgStart();
 	ASSERT_EQ(ed.UArgGet(), 1000000);
 }
+
+
+// std::regex recurses per matched character. Regex search and replace on a
+// very long line (minified JS/JSON) must not overflow the stack.
+TEST(Audit_Regex_LongLine_NoStackOverflow)
+{
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	Buffer &b  = h.Buf();
+	b.insert_text(0, 0, std::string(200000, 'a') + "\nshort\n");
+	b.SetCursor(0, 1);
+
+	ASSERT_TRUE(h.Exec(CommandId::RegexFindStart));
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "(a|b)*"));
+	ASSERT_TRUE(h.Exec(CommandId::Refresh));
+	ASSERT_TRUE(!ed.PromptActive());
+
+	regex_replace_all(h, "a+", "b");
+	ASSERT_EQ(h.Line(0), std::string("b"));
+	ASSERT_EQ(h.Line(1), std::string("short"));
+}