Support only short option

Fixes #47. Adds support for options that only have a short option.

include/cxxopts.hpp

@@ -851,7 +851,7 @@ namespace cxxopts
       ("--([[:alnum:]][-_[:alnum:]]+)(=(.*))?|-([[:alnum:]]+)");
 
     std::basic_regex<char> option_specifier
-      ("(([[:alnum:]]),)?([[:alnum:]][-_[:alnum:]]+)");
+      ("(([[:alnum:]]),)?([[:alnum:]][-_[:alnum:]]*)?");
 
     String
     format_option
@@ -985,8 +985,39 @@ OptionAdder::operator()
   const auto& s = result[2];
   const auto& l = result[3];
 
-  m_options.add_option(m_group, s.str(), l.str(), desc, value,
-    std::move(arg_help));
+  if (!s.length() && !l.length())
+  {
+    throw invalid_option_format_error(opts);
+  } else if (l.length() == 1 && s.length())
+  {
+    throw invalid_option_format_error(opts);
+  }
+
+  auto option_names = []
+  (
+    const std::sub_match<const char*>& s,
+    const std::sub_match<const char*>& l
+  )
+  {
+    if (l.length() == 1)
+    {
+      return std::make_tuple(l.str(), s.str());
+    }
+    else
+    {
+      return std::make_tuple(s.str(), l.str());
+    }
+  }(s, l);
+
+  m_options.add_option
+  (
+    m_group,
+    std::get<0>(option_names),
+    std::get<1>(option_names),
+    desc,
+    value,
+    std::move(arg_help)
+  );
 
   return *this;
 }

test/CMakeLists.txt

@@ -1,5 +1,5 @@
 if (CXXOPTS_BUILD_TESTS)
-    add_executable(options_test options.cpp)
+    add_executable(options_test main.cpp options.cpp)
     target_link_libraries(options_test cxxopts)
 
     if (MSVC)

test/options.cpp

@@ -1,4 +1,3 @@
-#define CATCH_CONFIG_MAIN
 #include "catch.hpp"
 
 #include <initializer_list>
@@ -80,6 +79,27 @@ TEST_CASE("Basic options", "[options]")
   CHECK(options.count("6") == 1);
 }
 
+TEST_CASE("Short options", "[options]")
+{
+  cxxopts::Options options("test_short", " - test short options");
+
+  options.add_options()
+    ("a", "a short option", cxxopts::value<std::string>());
+
+  Argv argv({"test_short", "-a", "value"});
+
+  auto actual_argv = argv.argv();
+  auto argc = argv.argc();
+
+  options.parse(argc, actual_argv);
+
+  CHECK(options.count("a") == 1);
+  CHECK(options["a"].as<std::string>() == "value");
+
+  REQUIRE_THROWS_AS(options.add_options()("", "nothing option"), 
+    cxxopts::invalid_option_format_error);
+}
+
 TEST_CASE("No positional", "[positional]")
 {
   cxxopts::Options options("test_no_positional",