diff --git a/lib/pry/pry_instance.rb b/lib/pry/pry_instance.rb
index 70a7e930..24a080a8 100644
--- a/lib/pry/pry_instance.rb
+++ b/lib/pry/pry_instance.rb
@@ -534,45 +534,54 @@ class Pry
     prompt_stack.size > 1 ? prompt_stack.pop : prompt
   end
 
-  if RUBY_VERSION =~ /1.9/ && RUBY_ENGINE == "ruby"
-    require 'ripper'
-
-    # Determine if a string of code is a valid Ruby expression.
-    # Ruby 1.9 uses Ripper, Ruby 1.8 uses RubyParser.
-    # @param [String] code The code to validate.
-    # @return [Boolean] Whether or not the code is a valid Ruby expression.
-    # @example
-    #   valid_expression?("class Hello") #=> false
-    #   valid_expression?("class Hello; end") #=> true
-    def valid_expression?(code)
-      !!Ripper::SexpBuilder.new(code).parse
+  # Determine if a string of code is a valid Ruby expression.
+  # @param [String] code The code to validate.
+  # @return [Boolean] Whether or not the code is a valid Ruby expression.
+  # @example
+  #   valid_expression?("class Hello") #=> false
+  #   valid_expression?("class Hello; end") #=> true
+  def valid_expression?(str)
+    if defined?(Rubinius::Melbourne19) && RUBY_VERSION =~ /^1\.9/
+      Rubinius::Melbourne19.parse_string(str, Pry.eval_path)
+    elsif defined?(Rubinius::Melbourne)
+      Rubinius::Melbourne.parse_string(str, Pry.eval_path)
+    else
+      catch(:valid) {
+        eval("BEGIN{throw :valid, true}\n#{str}", binding, Pry.eval_path)
+      }
     end
-
-  elsif RUBY_VERSION =~ /1.9/ && RUBY_ENGINE == 'jruby'
-
-    # JRuby doesn't have Ripper, so use its native parser for 1.9 mode.
-    def valid_expression?(code)
-      JRuby.parse(code)
-      true
-    rescue SyntaxError
+  rescue SyntaxError => e
+    if incomplete_user_input_exception?(e, caller)
       false
+    else
+      raise e
     end
+  end
 
-  else
-    require 'ruby_parser'
+  # Check whether the exception indicates that the user should input more.
+  #
+  # The first part of the check verifies that the exception was raised from
+  # the input to the eval, taking care not to be confused if the input contains
+  # an eval() with incomplete syntax.
+  #
+  # @param [SyntaxError] the exception object that was raised.
+  # @param [Array<String>] The stack frame of the function that executed eval.
+  # @return [Boolean]
+  #
+  def incomplete_user_input_exception?(ex, stack)
+    # deliberate use of ^ instead of \A, error messages can be two lines long.
+    from_pry_input = /^#{Regexp.escape(Pry.eval_path)}/
 
-    # Determine if a string of code is a valid Ruby expression.
-    # Ruby 1.9 uses Ripper, Ruby 1.8 uses RubyParser.
-    # @param [String] code The code to validate.
-    # @return [Boolean] Whether or not the code is a valid Ruby expression.
-    # @example
-    #   valid_expression?("class Hello") #=> false
-    #   valid_expression?("class Hello; end") #=> true
-    def valid_expression?(code)
-      # NOTE: we're using .dup because RubyParser mutates the input
-      RubyParser.new.parse(code.dup)
-      true
-    rescue Racc::ParseError, SyntaxError
+    return false unless SyntaxError === ex && ex.message =~ from_pry_input
+
+    case ex.message
+    when /unexpected (\$end|end-of-file|END_OF_FILE)/, # mri, jruby, ironruby
+        /unterminated (quoted string|string|regexp) meets end of file/, # "quoted string" is ironruby
+        /missing 'end' for/, /: expecting '[})\]]'$/, /can't find string ".*" anywhere before EOF/, /expecting keyword_end/ # rbx
+      backtrace = ex.backtrace
+      backtrace = backtrace.drop(1) if RUBY_VERSION =~ /^1\.8/ && RUBY_PLATFORM !~ /java/
+      backtrace.grep(from_pry_input).count <= stack.grep(from_pry_input).count
+    else
       false
     end
   end
diff --git a/test/helper.rb b/test/helper.rb
index 2a8e40dc..5c0107b4 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -180,3 +180,11 @@ CommandTester = Pry::CommandSet.new do
     output.puts arg
   end
 end
+
+# to help with tracking down bugs that cause an infinite loop in the test suite
+if ENV["SET_TRACE_FUNC"]
+  require 'set_trace' if defined?(RUBY_ENGINE) && RUBY_ENGINE =~ /rbx/ # gem install 'rbx-tracer'
+  set_trace_func proc { |event, file, line, id, binding, classname|
+     STDERR.printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
+  }
+end