diff --git a/include/cxxopts.hpp b/include/cxxopts.hpp
index f4fa7df..dab1351 100644
--- a/include/cxxopts.hpp
+++ b/include/cxxopts.hpp
@@ -884,7 +884,7 @@ namespace cxxopts
       ("--([[:alnum:]][-_[:alnum:]]+)(=(.*))?|-([[:alnum:]]+)");
 
     std::basic_regex<char> option_specifier
-      ("(([[:alnum:]]),)?[ ]*([[:alnum:]][-_[:alnum:]]*)?");
+      ("(([[:alnum:]]),)?\\s*([[:alnum:]][-_[:alnum:]]*)?");
 
     String
     format_option