⚡✅ Update all net-imap regexps to run in O(n)

The BIDI_FAILURE regexps all used `\g<name>` to define char classes and
then re-use them.  Unfortunately, ruby 3.2 can't compile that to run in
linear time.  The regexps could also be written using lookahead, but
that also wouldn't run in linear time.

The debug output gsub is simple to accomplish without negative lookahead
by using a block with gsub and checking `$'.empty?`.

`bin/check-regexps` was added for quick double-check, just in case the
tests aren't catching everything.

bin/check-regexps

@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# See also: test/net/imap/regexp_collector.rb
+# See also: test/net/imap/test_regexps.rb
+
+def traverse(m=Object, s=Set.new, &b)
+  m.constants(false).map{m.const_get _1 rescue nil}.select{_1 in Module}.each do
+    next if s.include?(_1); s << _1
+    b and b[_1]
+    traverse(_1, s, &b)
+  end
+end
+
+def collect_regexps = ObjectSpace
+  .each_object(Regexp)
+  .reject{Regexp.linear_time? _1}
+
+2.times{traverse}
+before = collect_regexps
+
+$LOAD_PATH.unshift "./lib"
+require 'net/imap'
+2.times{traverse}
+traverse(Net::IMAP) { puts _1.name }
+after = collect_regexps - before
+p before: before.count, count: after.count, after:;

lib/net/imap.rb

@@ -2443,8 +2443,8 @@ def put_string(str)
         if @debug_output_bol
           $stderr.print("C: ")
         end
-        $stderr.print(str.gsub(/\n(?!\z)/n, "\nC: "))
-        if /\r\n\z/n.match(str)
+        $stderr.print(str.gsub(/\n/n) { $'.empty? ? $& : "\nC: " })
+        if /\n\z/n.match(str)
           @debug_output_bol = true
         else
           @debug_output_bol = false