diff --git a/src/options.cpp b/src/options.cpp
index ea1093e..59000e9 100644
--- a/src/options.cpp
+++ b/src/options.cpp
@@ -26,6 +26,8 @@
 #include <boost/program_options/positional_options.hpp>
 #include <boost/program_options/value_semantic.hpp>
 #include <boost/program_options/variables_map.hpp>
+#include <fmt/format.h>
+#include <fmt/ostream.h> // For compatibility with fmt 4.
 
 #include <cstdint>
 #include <cstdlib>
@@ -42,6 +44,7 @@ namespace epubgrep::options
 namespace po = boost::program_options;
 
 using boost::locale::translate;
+using fmt::format;
 using std::cout;
 
 options parse_options(int argc, char *argv[])
@@ -243,4 +246,66 @@ options parse_again(const po::variables_map &vm)
     return opts;
 }
 
+std::ostream &operator<<(std::ostream &out, const options &opts)
+{
+    const std::string regex_kind{[&opts]
+                                 {
+                                     switch (opts.regex)
+                                     {
+                                     case regex_kind::basic:
+                                     {
+                                         return "basic";
+                                         break;
+                                     }
+                                     case regex_kind::extended:
+                                     {
+                                         return "extended";
+                                         break;
+                                     }
+                                     case regex_kind::perl:
+                                     {
+                                         return "perl";
+                                         break;
+                                     }
+                                     }
+                                     return "error";
+                                 }()};
+    out << format("help={0:} version={1:} regex={2:s} grep={3:} "
+                  "ignore_case={4:} ",
+                  opts.help, opts.version, regex_kind, opts.grep,
+                  opts.ignore_case);
+
+    out << "regexp={";
+    for (const auto &regexp : opts.regexp)
+    {
+        if (regexp != *opts.regexp.begin())
+        {
+            out << ", ";
+        }
+        out << '"' << regexp << '"';
+    }
+    out << "} ";
+
+    out << format("raw={0:} context={1:d} nocolor={2:} no_fn_fs={3:} "
+                  "no_fn_epub={4:} recursive={5:} dereference_recursive={6:} ",
+                  opts.raw, opts.context, opts.nocolor, opts.no_fn_fs,
+                  opts.no_fn_epub, opts.recursive, opts.dereference_recursive);
+
+    out << "input_file={";
+    for (const auto &input_file : opts.input_file)
+    {
+        if (input_file != *opts.input_file.begin())
+        {
+            out << ", ";
+        }
+        out << '"' << input_file << '"';
+    }
+    out << "} ";
+
+    out << format("ignore_archive={0:} debug={1:}", opts.ignore_archive_errors,
+                  opts.debug);
+
+    return out;
+}
+
 } // namespace epubgrep::options
diff --git a/src/options.hpp b/src/options.hpp
index 68506b5..45ea893 100644
--- a/src/options.hpp
+++ b/src/options.hpp
@@ -23,6 +23,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <ostream>
 #include <string>
 #include <vector>
 
@@ -56,6 +57,9 @@ struct options
     std::vector<std::string> input_file;
     bool ignore_archive_errors{false};
     bool debug{false};
+
+    //! For the debug output.
+    friend std::ostream &operator<<(std::ostream &out, const options &opts);
 };
 
 //! Parse options and return them.