Regex: optional PCRE2 engine behind kte::Regex

RegexEngine.h/.cc wrap the regex engine for search, regex replace-all and
the terminal and ImGui renderers' match highlighting:

- With KTE_USE_PCRE2 (default ON; used when pkg-config finds libpcre2-8,
  linked as -lpcre2-8) patterns are compiled by PCRE2 with its JIT and
  matched under a match limit: catastrophic backtracking stops with a
  status note instead of hanging the editor, and matching does not
  recurse on the C stack (no large-stack thread; the per-keystroke and
  per-frame line limit is 1 MiB instead of 2,000 bytes). Otherwise
  std::regex is used as before.
- Replacement strings follow std::regex_replace (ECMAScript as libstdc++
  reads it) with either engine: the PCRE2 path expands $&, $n, $`, $',
  $$ itself and steps over empty matches the same way.
- Regex search walks lines over the materialized text instead of calling
  GetLineView per row. The terminal renderer compiles the pattern once
  per frame (it compiled it for every row).
- Only one checkpoint per journal is queued at a time: whole-file edits
  in a row on a large buffer each queued a full copy of the buffer.
- default.nix and the Dockerfile add pcre2 and pkg-config.

On 26 MB: regex keystroke 1.6 -> 0.14 ms, next 62 -> 6 ms, replace-all
929 -> 438 ms; a rarely matching regex over 105 MB: 1.55 s -> 0.10 s.

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

RegexEngine.h

@@ -0,0 +1,68 @@
+/*
+ * RegexEngine.h - the regex engine behind search, replace and match
+ * highlighting.
+ *
+ * With KTE_USE_PCRE2 (the default when libpcre2-8 is found) patterns are
+ * compiled by PCRE2 with its JIT: typically one to two orders of magnitude
+ * faster than std::regex, no recursion on the C stack, and a match limit
+ * that stops catastrophic backtracking (e.g. "^(a+)+$") instead of hanging
+ * the editor. Otherwise std::regex (ECMAScript) is used, run on a large
+ * stack (see RegexGuard.h). The syntax both accept covers ordinary use;
+ * PCRE2 additionally has lookbehind, possessive quantifiers and the like.
+ *
+ * Replacement strings use ECMAScript syntax with either engine, as
+ * std::regex_replace reads it: $& (the match), $1..$99 (groups), $` (the
+ * text since the previous match), $' (the rest of the line), $$ (a dollar).
+ */
+#pragma once
+
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <string_view>
+
+namespace kte {
+class Regex {
+public:
+	Regex();
+
+	~Regex();
+
+	Regex(Regex &&other) noexcept;
+
+	Regex &operator=(Regex &&other) noexcept;
+
+	Regex(const Regex &) = delete;
+
+	Regex &operator=(const Regex &) = delete;
+
+	// Compile; on failure err describes the problem and Valid() is false.
+	bool Compile(std::string_view pattern, std::string &err);
+
+	[[nodiscard]] bool Valid() const;
+
+	// The first match in `subject` starting at byte `from` or later; `from`
+	// may be inside the subject (anchors and lookbehind see what precedes
+	// it). False if there is none, or if the engine gave up (LimitHit()).
+	bool Search(std::string_view subject, std::size_t from, std::size_t &pos, std::size_t &len) const;
+
+	// Every non-overlapping match replaced by `fmt` (ECMAScript syntax, see
+	// above); an empty match is not repeated at the same position.
+	[[nodiscard]] std::string ReplaceAll(std::string_view subject, std::string_view fmt) const;
+
+	// Whether the last Search/ReplaceAll stopped at the engine's match limit.
+	[[nodiscard]] bool LimitHit() const;
+
+	// "PCRE2" or "std::regex".
+	static const char *EngineName();
+
+	// The longest line search-as-you-type and match highlighting run the
+	// regex over (per keystroke/frame); std::regex can be quadratic in line
+	// length with no way to interrupt it, PCRE2 is bounded by its limit.
+	static std::size_t IncrementalLineLimit();
+
+private:
+	struct Impl;
+	std::unique_ptr<Impl> impl_;
+};
+} // namespace kte