Add support for regex flag to toggle dot-matches-newline

doc/pages/regex.asciidoc

@@ -156,7 +156,9 @@ Some modifiers can control the matching behavior of the atoms following
 them:
 
 * `(?i)` enables case-insensitive matching
-* `(?I)` disables case-insensitive matching
+* `(?I)` disables case-insensitive matching (default)
+* `(?s)` enables dot-matches-newline (default)
+* `(?S)` disables dot-matches-newline
 
 == Quoting
 

src/regex_impl.cc

@@ -200,14 +200,23 @@ private:
     {
         auto it = m_pos.base();
         if (m_regex.end() - it >= 4 and *it++ == '(' and *it++ == '?')
+        {
+            while (true)
             {
                 auto m = *it++;
-            if ((m != 'i' and m != 'I') or *it++ != ')')
-                return false;
+                if (m == 'i' or m == 'I')
                     m_ignore_case = (m == 'i');
+                else if (m == 's' or m == 'S')
+                    m_dot_maches_newline = (m == 's');
+                else if (m == ')')
+                {
                     m_pos = Iterator{it, m_regex};
                     return true;
                 }
+                else
+                    return false;
+            }
+        }
         return false;
     }
 
@@ -274,7 +283,19 @@ private:
 
         switch (const Codepoint cp = *m_pos)
         {
-            case '.': ++m_pos; return new_node(ParsedRegex::AnyChar);
+            case '.':
+                ++m_pos;
+                if (m_dot_maches_newline)
+                    return new_node(ParsedRegex::AnyChar);
+                else
+                {
+                    CharacterClass c;
+                    c.negative = true;
+                    c.ranges.push_back({ '\n', '\n' });
+                    auto class_id = m_parsed_regex.character_classes.size();
+                    m_parsed_regex.character_classes.push_back(std::move(c));
+                    return new_node(ParsedRegex::Class, class_id);
+                }
             case '(':
             {
                 auto captures = [this, it = (++m_pos).base()]() mutable {
@@ -594,6 +615,7 @@ private:
     StringView m_regex;
     Iterator m_pos;
     bool m_ignore_case = false;
+    bool m_dot_maches_newline = true;
 
     static constexpr struct CharacterClassEscape {
         Codepoint cp;