Linkparser: Use callbacks, so app can influence how the HTML gets rendered exactly

In particular, this allows making dynamic decisions: Some links may
for example require a different CSS class or so, therefore a static
hardcoded template is not enough for those cases.

include/maddy/callbackreplacer.h

@@ -0,0 +1,36 @@
+/*
+ * This project is licensed under the MIT license. For more information see the
+ * LICENSE file.
+ */
+#pragma once
+
+#include <string>
+#include <regex>
+#include <functional>
+
+namespace maddy {
+
+inline std::string regex_callback_replacer(std::regex &regex, const std::string &input,
+                                           std::function<std::string(std::smatch &)> &callback)
+{
+  std::string result;
+  auto tagsbegin = std::sregex_iterator(input.begin(), input.end(), regex);
+  auto tagsend = std::sregex_iterator();
+  auto matchbegin = 0;
+  for(std::sregex_iterator i = tagsbegin; i != tagsend; ++i)
+  {
+    std::smatch match = *i;
+
+    auto matchlength = match.length(0);
+    auto matchpos = match.position();
+
+    result += input.substr(matchbegin, matchpos - matchbegin);
+    result += callback(match);
+    matchbegin = matchpos + matchlength;
+  }
+  result += input.substr(matchbegin);
+  return result;
+}
+
+}
+

include/maddy/linkparser.h

@@ -10,6 +10,7 @@
 #include <regex>
 
 #include "maddy/lineparser.h"
+#include "maddy/callbackreplacer.h"
 
 // -----------------------------------------------------------------------------
 
@@ -26,6 +27,16 @@ namespace maddy {
  */
 class LinkParser : public LineParser
 {
+private:
+  std::function<std::string(std::smatch &)> callback = [](std::smatch & match){
+    std::string inner = match.str(1);
+    std::string link = match.str(2);
+    return "<a href=\"" + link + "\">" + inner + "</a>";
+  };
+
+  std::regex re = std::regex("\\[([^\\]]*)\\]\\(([^\\]]*)\\)");
+
+
 public:
   /**
    * Parse
@@ -41,10 +52,12 @@ public:
   void
   Parse(std::string& line) override
   {
-    static std::regex re("\\[([^\\]]*)\\]\\(([^\\]]*)\\)");
-    static std::string replacement = "<a href=\"$2\">$1</a>";
+    line = regex_callback_replacer(re,line,callback);
+  }
 
-    line = std::regex_replace(line, re, replacement);
+  void setCallback(std::function<std::string(std::smatch &)> callback)
+  {
+    this->callback = callback;
   }
 }; // class LinkParser