diff --git a/.travis.yml b/.travis.yml
index 99366263c..94a99fb79 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,6 +16,7 @@ rvm:
   - "2.4"
   - "2.5"
   - "2.6"
+  - "2.7"
   # - ruby-head
   # - jruby
 
diff --git a/lib/yard/parser/ruby/legacy/irb/slex.rb b/lib/yard/parser/ruby/legacy/irb/slex.rb
new file mode 100644
index 000000000..2f363bded
--- /dev/null
+++ b/lib/yard/parser/ruby/legacy/irb/slex.rb
@@ -0,0 +1,276 @@
+# frozen_string_literal: false
+#
+#   irb/slex.rb - simple lex analyzer
+#   	$Release Version: 0.9.6$
+#   	$Revision$
+#   	by Keiju ISHITSUKA(keiju@ruby-lang.org)
+#
+# --
+#
+#
+#
+
+require "irb/notifier"
+
+# :stopdoc:
+module IRB
+  class SLex
+    DOUT = Notifier::def_notifier("SLex::")
+    D_WARN = DOUT::def_notifier(1, "Warn: ")
+    D_DEBUG = DOUT::def_notifier(2, "Debug: ")
+    D_DETAIL = DOUT::def_notifier(4, "Detail: ")
+
+    DOUT.level = Notifier::D_NOMSG
+
+    def initialize
+      @head = Node.new("")
+    end
+
+    def def_rule(token, preproc = nil, postproc = nil, &block)
+      D_DETAIL.pp token
+
+      postproc = block if block_given?
+      create(token, preproc, postproc)
+    end
+
+    def def_rules(*tokens, &block)
+      if block_given?
+        p = block
+      end
+      for token in tokens
+        def_rule(token, nil, p)
+      end
+    end
+
+    def preproc(token, proc)
+      node = search(token)
+      node.preproc=proc
+    end
+
+    # need a check?
+    def postproc(token)
+      node = search(token, proc)
+      node.postproc=proc
+    end
+
+    def search(token)
+      @head.search(token.split(//))
+    end
+
+    def create(token, preproc = nil, postproc = nil)
+      @head.create_subnode(token.split(//), preproc, postproc)
+    end
+
+    def match(token)
+      case token
+      when Array
+      when String
+        return match(token.split(//))
+      else
+        return @head.match_io(token)
+      end
+      ret = @head.match(token)
+      D_DETAIL.exec_if{D_DETAIL.printf "match end: %s:%s\n", ret, token.inspect}
+      ret
+    end
+
+    def inspect
+      format("<SLex: @head = %s>", @head.inspect)
+    end
+
+    #----------------------------------------------------------------------
+    #
+    #   class Node -
+    #
+    #----------------------------------------------------------------------
+    class Node
+      # if postproc is nil, this node is an abstract node.
+      # if postproc is non-nil, this node is a real node.
+      def initialize(preproc = nil, postproc = nil)
+        @Tree = {}
+        @preproc = preproc
+        @postproc = postproc
+      end
+
+      attr_accessor :preproc
+      attr_accessor :postproc
+
+      def search(chrs, opt = nil)
+        return self if chrs.empty?
+        ch = chrs.shift
+        if node = @Tree[ch]
+          node.search(chrs, opt)
+        else
+          if opt
+            chrs.unshift ch
+            self.create_subnode(chrs)
+          else
+            raise "node nothing"
+          end
+        end
+      end
+
+      def create_subnode(chrs, preproc = nil, postproc = nil)
+        if chrs.empty?
+          if @postproc
+            D_DETAIL.pp node
+            raise "node already exists"
+          else
+            D_DEBUG.puts "change abstract node to real node."
+            @preproc = preproc
+            @postproc = postproc
+          end
+          return self
+        end
+
+        ch = chrs.shift
+        if node = @Tree[ch]
+          if chrs.empty?
+            if node.postproc
+              DebugLogger.pp node
+              DebugLogger.pp self
+              DebugLogger.pp ch
+              DebugLogger.pp chrs
+              raise "node already exists"
+            else
+              D_WARN.puts "change abstract node to real node"
+              node.preproc = preproc
+              node.postproc = postproc
+            end
+          else
+            node.create_subnode(chrs, preproc, postproc)
+          end
+        else
+          if chrs.empty?
+            node = Node.new(preproc, postproc)
+          else
+            node = Node.new
+            node.create_subnode(chrs, preproc, postproc)
+          end
+          @Tree[ch] = node
+        end
+        node
+      end
+
+      #
+      # chrs: String
+      #       character array
+      #       io must have getc()/ungetc(); and ungetc() must be
+      #       able to be called arbitrary number of times.
+      #
+      def match(chrs, op = "")
+        D_DETAIL.print "match>: ", chrs, "op:", op, "\n"
+        if chrs.empty?
+          if @preproc.nil? || @preproc.call(op, chrs)
+            DOUT.printf(D_DETAIL, "op1: %s\n", op)
+            @postproc.call(op, chrs)
+          else
+            nil
+          end
+        else
+          ch = chrs.shift
+          if node = @Tree[ch]
+            if ret = node.match(chrs, op+ch)
+              return ret
+            else
+              chrs.unshift ch
+              if @postproc and @preproc.nil? || @preproc.call(op, chrs)
+                DOUT.printf(D_DETAIL, "op2: %s\n", op.inspect)
+                ret = @postproc.call(op, chrs)
+                return ret
+              else
+                return nil
+              end
+            end
+          else
+            chrs.unshift ch
+            if @postproc and @preproc.nil? || @preproc.call(op, chrs)
+              DOUT.printf(D_DETAIL, "op3: %s\n", op)
+              @postproc.call(op, chrs)
+              return ""
+            else
+              return nil
+            end
+          end
+        end
+      end
+
+      def match_io(io, op = "")
+        if op == ""
+          ch = io.getc
+          if ch == nil
+            return nil
+          end
+        else
+          ch = io.getc_of_rests
+        end
+        if ch.nil?
+          if @preproc.nil? || @preproc.call(op, io)
+            D_DETAIL.printf("op1: %s\n", op)
+            @postproc.call(op, io)
+          else
+            nil
+          end
+        else
+          if node = @Tree[ch]
+            if ret = node.match_io(io, op+ch)
+              ret
+            else
+              io.ungetc ch
+              if @postproc and @preproc.nil? || @preproc.call(op, io)
+                DOUT.exec_if{D_DETAIL.printf "op2: %s\n", op.inspect}
+                @postproc.call(op, io)
+              else
+                nil
+              end
+            end
+          else
+            io.ungetc ch
+            if @postproc and @preproc.nil? || @preproc.call(op, io)
+              D_DETAIL.printf("op3: %s\n", op)
+              @postproc.call(op, io)
+            else
+              nil
+            end
+          end
+        end
+      end
+    end
+  end
+end
+# :startdoc:
+
+if $0 == __FILE__
+  case $1
+  when "1"
+    tr = SLex.new
+    print "0: ", tr.inspect, "\n"
+    tr.def_rule("=") {print "=\n"}
+    print "1: ", tr.inspect, "\n"
+    tr.def_rule("==") {print "==\n"}
+    print "2: ", tr.inspect, "\n"
+
+    print "case 1:\n"
+    print tr.match("="), "\n"
+    print "case 2:\n"
+    print tr.match("=="), "\n"
+    print "case 3:\n"
+    print tr.match("=>"), "\n"
+
+  when "2"
+    tr = SLex.new
+    print "0: ", tr.inspect, "\n"
+    tr.def_rule("=") {print "=\n"}
+    print "1: ", tr.inspect, "\n"
+    tr.def_rule("==", proc{false}) {print "==\n"}
+    print "2: ", tr.inspect, "\n"
+
+    print "case 1:\n"
+    print tr.match("="), "\n"
+    print "case 2:\n"
+    print tr.match("=="), "\n"
+    print "case 3:\n"
+    print tr.match("=>"), "\n"
+  end
+  exit
+end
diff --git a/lib/yard/parser/ruby/legacy/ruby_lex.rb b/lib/yard/parser/ruby/legacy/ruby_lex.rb
index 412b2d042..0a843d0f5 100644
--- a/lib/yard/parser/ruby/legacy/ruby_lex.rb
+++ b/lib/yard/parser/ruby/legacy/ruby_lex.rb
@@ -1,5 +1,4 @@
-require "e2mmap"
-require "irb/slex"
+require_relative 'irb/slex'
 
 module YARD
   module Parser::Ruby::Legacy
@@ -129,7 +128,7 @@ def Token(token, value = nil) # rubocop:disable Style/MethodName
         when String, Symbol
           source = token.is_a?(String) ? TkReading2Token : TkSymbol2Token
           if (tk = source[token]).nil?
-            IRB.fail TkReading2TokenNoKey, token
+            raise "no key #{token}"
           end
           tk = Token(tk[0], value)
         else
@@ -285,7 +284,7 @@ def self.def_token(token_n, super_token = Token, reading = nil, *opts)
 
         if reading
           if TkReading2Token[reading]
-            IRB.fail TkReading2TokenDuplicateError, token_n, reading
+            raise "duplicate #{token_n} #{reading}"
           end
           if opts.empty?
             TkReading2Token[reading] = [token_c]
@@ -425,14 +424,6 @@ def divert_read_from(reserve)
 
       # end of nested class BufferedReader
 
-      extend Exception2MessageMapper
-      def_exception(:AlreadyDefinedToken, "Already defined token(%s)")
-      def_exception(:TkReading2TokenNoKey, "key nothing(key='%s')")
-      def_exception(:TkSymbol2TokenNoKey, "key nothing(key='%s')")
-      def_exception(:TkReading2TokenDuplicateError,
-        "key duplicate(token_n='%s', key='%s')")
-      def_exception(:SyntaxError, "%s")
-
       include RubyToken
       include IRB