From a973526c050fec044ffd7ceeba0ac8e8a1fed299 Mon Sep 17 00:00:00 2001
From: Takashi Kokubun <takashikkbn@gmail.com>
Date: Thu, 11 Dec 2025 14:41:36 -0800
Subject: tool/format-release: Fix the tag format for Ruby 4.0+

---
 tool/format-release | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/tool/format-release b/tool/format-release
index b7ad74a095..b38263f9f4 100755
--- a/tool/format-release
+++ b/tool/format-release
@@ -62,19 +62,18 @@ eom
     if z != 0
       prev_tag = nil
     elsif y != 0
-      prev_tag = "v#{x}_#{y-1}_0"
       prev_ver = "#{x}.#{y-1}.0"
+      prev_tag = version_tag(prev_ver)
     else # y == 0 && z == 0
       case x
       when 3
-        prev_tag = "v2_7_0"
         prev_ver = "2.7.0"
       when 4
-        prev_tag = "v3_4_0"
         prev_ver = "3.4.0"
       else
         raise "it doesn't know what is the previous version of '#{version}'"
       end
+      prev_tag = version_tag(prev_ver)
     end
 
     uri = "https://cache.ruby-lang.org/pub/tmp/ruby-info-#{version}-draft.yml"
@@ -95,7 +94,7 @@ eom
 
     if prev_tag
       # show diff shortstat
-      tag = "v#{version.gsub(/[.\-]/, '_')}"
+      tag = version_tag(version)
       stat = `git -C #{rubydir} diff -l0 --shortstat #{prev_tag}..#{tag}`
       files_changed, insertions, deletions = stat.scan(/\d+/)
     end
@@ -189,7 +188,7 @@ eom
     if /\.0(?:-\w+)?\z/ =~ ver
       # preview, rc, or first release
       entry <<= <<eom
-  tag: v#{ver.tr('.-', '_')}
+  tag: #{version_tag(ver)}
   stats:
     files_changed: #{files_changed}
     insertions: #{insertions}
@@ -248,6 +247,15 @@ eom
       puts line
     end
   end
+
+  def self.version_tag(version)
+    major_version = Integer(version.split('.', 2)[0])
+    if major_version >= 4
+      "v#{version}"
+    else
+      "v#{version.tr('.-', '_')}"
+    end
+  end
 end
 
 def main
-- 
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