Add a disabled wrapper for using std regex instead of boost

src/regex.hh

@@ -3,26 +3,50 @@
 
 #include "string.hh"
 
-#if 0
+#ifdef KAK_USE_STDREGEX
 #include <regex>
-namespace kak_regex_ns = std;
 #else
 #include <boost/regex.hpp>
-namespace kak_regex_ns = boost;
 #endif
 
 namespace Kakoune
 {
 
-using Regex = kak_regex_ns::regex;
+#ifdef KAK_USE_STDREGEX
+// Regex that keeps track of its string representation
+struct Regex : std::regex
+{
+    Regex() = default;
+
+    explicit Regex(StringView re, flag_type flags = ECMAScript)
+        : std::regex(re.begin(), re.end(), flags), m_str(re) {}
+
+    template<typename Iterator>
+    Regex(Iterator begin, Iterator end, flag_type flags = ECMAScript)
+        : std::regex(begin, end, flags), m_str(begin, end) {}
+
+    bool empty() const { return m_str.empty(); }
+    bool operator==(const Regex& other) { return m_str == other.m_str; }
+    bool operator!=(const Regex& other) { return m_str != other.m_str; }
+
+    StringView str() const { return m_str; }
+
+private:
+    String m_str;
+};
+namespace regex_ns = std;
+#else
+namespace regex_ns = boost;
+using Regex = boost::regex;
+#endif
 
 template<typename Iterator>
-using RegexIterator = kak_regex_ns::regex_iterator<Iterator>;
+using RegexIterator = regex_ns::regex_iterator<Iterator>;
 
 template<typename Iterator>
-using MatchResults = kak_regex_ns::match_results<Iterator>;
+using MatchResults = regex_ns::match_results<Iterator>;
 
-using RegexError = kak_regex_ns::regex_error;
+using RegexError = regex_ns::regex_error;
 
 String option_to_string(const Regex& re);
 void option_from_string(StringView str, Regex& re);