diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index 2f70f46ad3..e41f6887aa 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -99,7 +99,7 @@ def parse_args(argv = ARGV)
     case v
     when /\AINSTALL[-_]([-\w]+)=(.*)/
       argv.unshift("--#{$1.tr('_', '-')}=#{$2}")
-    when /\A\w[-\w+]*=\z/
+    when /\A\w[-\w]*=/
       mflags << v
     when /\A\w[-\w+]*\z/
       $install << v.intern