diff --git a/ext/ripper/lib/ripper/lexer.rb b/ext/ripper/lib/ripper/lexer.rb
index ece71dd466442a..344328482b83a8 100644
--- a/ext/ripper/lib/ripper/lexer.rb
+++ b/ext/ripper/lib/ripper/lexer.rb
@@ -207,14 +207,14 @@ def compile(pattern)
     end
 
     def map_tokens(tokens)
-      tokens.map {|pos,type,str| map_token(type.to_s.sub(/\Aon_/,'')) }.join
+      tokens.map {|pos,type,str| map_token(type.to_s.delete_prefix('on_')) }.join
     end
 
     MAP = {}
     seed = ('a'..'z').to_a + ('A'..'Z').to_a + ('0'..'9').to_a
     SCANNER_EVENT_TABLE.each do |ev, |
       raise CompileError, "[RIPPER FATAL] too many system token" if seed.empty?
-      MAP[ev.to_s.sub(/\Aon_/,'')] = seed.shift
+      MAP[ev.to_s.delete_prefix('on_')] = seed.shift
     end
 
     def map_token(tok)
diff --git a/lib/cgi/core.rb b/lib/cgi/core.rb
index 3fb8dbab542333..c45f9b3a9e8ed4 100644
--- a/lib/cgi/core.rb
+++ b/lib/cgi/core.rb
@@ -421,7 +421,7 @@ def CGI::parse(query)
   module QueryExtension
 
     %w[ CONTENT_LENGTH SERVER_PORT ].each do |env|
-      define_method(env.sub(/^HTTP_/, '').downcase) do
+      define_method(env.delete_prefix('HTTP_').downcase) do
         (val = env_table[env]) && Integer(val)
       end
     end
@@ -434,7 +434,7 @@ module QueryExtension
         HTTP_ACCEPT HTTP_ACCEPT_CHARSET HTTP_ACCEPT_ENCODING
         HTTP_ACCEPT_LANGUAGE HTTP_CACHE_CONTROL HTTP_FROM HTTP_HOST
         HTTP_NEGOTIATE HTTP_PRAGMA HTTP_REFERER HTTP_USER_AGENT ].each do |env|
-      define_method(env.sub(/^HTTP_/, '').downcase) do
+      define_method(env.delete_prefix('HTTP_').downcase) do
         env_table[env]
       end
     end
diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 355ea171ee8adc..0d9d3d9f387d2f 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -2264,7 +2264,7 @@ def create_makefile(target, srcprefix = nil)
     origdef ||= ''
 
     if $extout and $INSTALLFILES
-      $cleanfiles.concat($INSTALLFILES.collect {|files, dir|File.join(dir, files.sub(/\A\.\//, ''))})
+      $cleanfiles.concat($INSTALLFILES.collect {|files, dir|File.join(dir, files.delete_prefix('./'))})
       $distcleandirs.concat($INSTALLFILES.collect {|files, dir| dir})
     end
 
diff --git a/lib/net/protocol.rb b/lib/net/protocol.rb
index 037fcd83588698..71378af8044386 100644
--- a/lib/net/protocol.rb
+++ b/lib/net/protocol.rb
@@ -311,7 +311,7 @@ def each_message_chunk
       read_bytes = 0
       while (line = readuntil("\r\n")) != ".\r\n"
         read_bytes += line.size
-        yield line.sub(/\A\./, '')
+        yield line.delete_prefix('.')
       end
       LOG_on()
       LOG "read message (#{read_bytes} bytes)"
diff --git a/lib/optparse.rb b/lib/optparse.rb
index c6e652971ea8a8..5cdcabf4a75571 100644
--- a/lib/optparse.rb
+++ b/lib/optparse.rb
@@ -1595,7 +1595,7 @@ def parse_in_order(argv = default_argv, setter = nil, &nonopt)  # :nodoc:
               begin
                 sw, = complete(:short, opt)
                 # short option matched.
-                val = arg.sub(/\A-/, '')
+                val = arg.delete_prefix('-')
                 has_arg = true
               rescue InvalidOption
                 # if no short options match, try completion with long
diff --git a/lib/un.rb b/lib/un.rb
index d109a3417bb002..14f5b10feccffa 100644
--- a/lib/un.rb
+++ b/lib/un.rb
@@ -47,7 +47,7 @@ def setup(options = "", *long_options)
     end
     long_options.each do |s|
       opt_name, arg_name = s.split(/(?=[\s=])/, 2)
-      opt_name.sub!(/\A--/, '')
+      opt_name.delete_prefix!('--')
       s = "--#{opt_name.gsub(/([A-Z]+|[a-z])([A-Z])/, '\1-\2').downcase}#{arg_name}"
       puts "#{opt_name}=>#{s}" if $DEBUG
       opt_name = opt_name.intern