diff --git a/tool/gen_dummy_probes.rb b/tool/gen_dummy_probes.rb
index c19f9d53cd..05e724fed8 100755
--- a/tool/gen_dummy_probes.rb
+++ b/tool/gen_dummy_probes.rb
@@ -2,7 +2,6 @@
 # -*- coding: us-ascii -*-
 
 text = ARGF.read
-text.gsub!(/^(?!#)(.*)/){$1.upcase}
 
 # remove comments
 text.gsub!(%r'(?:^ *)?/\*.*?\*/\n?'m, '')
@@ -11,18 +10,20 @@ text.gsub!(%r'(?:^ *)?/\*.*?\*/\n?'m, '')
 text.gsub!(/^#pragma.*\n/, '')
 
 # replace the provider section with the start of the header file
-text.gsub!(/PROVIDER RUBY \{/, "#ifndef\t_PROBES_H\n#define\t_PROBES_H\n#define DTRACE_PROBES_DISABLED 1\n")
+text.gsub!(/provider ruby \{/, "#ifndef\t_PROBES_H\n#define\t_PROBES_H\n#define DTRACE_PROBES_DISABLED 1\n")
 
 # finish up the #ifndef sandwich
 text.gsub!(/\};/, "\n#endif\t/* _PROBES_H */")
 
-text.gsub!(/__/, '_')
-
-text.gsub!(/\((.+?)(?=\);)/) {
-  "(arg" << (0..$1.count(',')).to_a.join(", arg")
+# expand probes to DTRACE macros
+text.gsub!(/^ *probe ([^\(]*)\(([^\)]*)\);/) {
+  name, args = $1, $2
+  name.upcase!
+  name.gsub!(/__/, '_')
+  args.gsub!(/(\A|, *)[^,]*\b(?=\w+(?=,|\z))/, '\1')
+  "#define RUBY_DTRACE_#{name}_ENABLED() 0\n" \
+  "#define RUBY_DTRACE_#{name}(#{args}) do {} while (0)"
 }
 
-text.gsub!(/ *PROBE ([^\(]*)(\([^\)]*\));/, "#define RUBY_DTRACE_\\1_ENABLED() 0\n#define RUBY_DTRACE_\\1\\2\ do \{ \} while\(0\)")
 puts "/* -*- c -*- */"
 print text
-