diff --git a/interpreter.rb b/interpreter.rb
new file mode 100644
index 0000000..ecf5945
--- /dev/null
+++ b/interpreter.rb
@@ -0,0 +1,227 @@
+# An interpreter as described by Jack Crenshaw in his famous book
+# "Let's Build a Compiler".  At least in the beginning, this code will
+# closely reflect the Pascal code written by Jack.  Over time it may
+# become more idiomatic, however this is an academic exercise.
+#
+# sjs
+# may 2009
+
+class ParseError < StandardError; end 
+
+class Interpreter
+  def initialize(input=STDIN)
+    @look = ''                   # next lookahead char
+    @input = input               # stream to read from
+    @vars = Hash.new(0)
+
+    # seed the lexer
+    get_char
+  end
+
+  def run
+    statement until eof? || @look == '.'
+  end
+
+  # Read the next character from the input stream
+  def get_char
+    @look = if @input.eof?
+              nil
+            else
+              @input.readbyte.chr
+            end
+  end
+
+  # Report error and halt
+  def abort(msg)
+    raise ParseError, msg
+  end
+
+  # Report what was expected
+  def expected(what)
+    msg = if eof?
+            "Premature end of file, expected: #{what}."
+          else
+            "Expected: #{what}, got: #{@look}."
+          end
+    abort(msg)
+  end
+
+
+  # Define a variable.
+  def define(name, value=nil)
+    abort("already defined '#{name}'") if @vars.has_key?(name)
+    set(name, value)
+  end
+
+  # Set the value of a variable.
+  def set(name, value)
+    @vars[name] = value
+  end
+
+  # Retrieve the value of a variable.
+  def get(name)
+    abort("undefined variable '#{name}'") unless @vars.has_key?(name)
+    @vars[name]
+  end
+
+
+  # Recognize an alphabetical character.
+  def alpha?(char)
+    ('A'..'Z') === char.upcase
+  end
+
+  # Recognize a decimal digit.
+  def digit?(char)
+    ('0'..'9') === char
+  end
+
+  # Recognize an alphanumeric character.
+  def alnum?(char)
+    alpha?(char) || digit?(char)
+  end
+
+  def whitespace?(char)
+    char == ' ' || char == '\t'
+  end
+
+
+  # Match a specific input character.
+  def match(char)
+    expected("'#{char}'") unless @look == char
+    get_char
+    skip_whitespace
+  end
+
+  # Parse zero or more consecutive characters for which the test is
+  # true.
+  def many(test)
+    token = ''
+    while test.call(@look)
+      token << @look
+      get_char
+    end
+    skip_whitespace
+    token
+  end
+
+  # Get an identifier.
+  def get_name
+    expected('identifier') unless alpha?(@look)
+    many(method(:alpha?))
+  end
+
+  # Get a number.
+  def get_num
+    expected('integer') unless digit?(@look)
+    many(method(:digit?)).to_i
+  end
+
+  # Skip all leading whitespace.
+  def skip_whitespace
+    get_char while whitespace?(@look)
+  end
+
+
+
+  # Parse and evaluate a factor.
+  def factor
+    if @look == '('
+      match('(')
+      value = expression
+      match(')')
+    elsif alpha?(@look)
+      value = get(get_name)
+    else
+      value = get_num
+    end
+    value
+  end
+
+  # Parse and evaluate a term.
+  def term
+    value = factor
+    while mulop?
+      case @look
+      when '*'
+        match('*')
+        value *= factor
+      when '/'
+        match('/')
+        value /= factor
+      end
+    end
+    value
+  end
+
+  # Parse and evaluate a mathematical expression.
+  def expression
+    # Fake unary plus and minus by prepending a 0 to them.
+    value = if addop? then 0 else term end
+    while addop?
+      case @look
+      when '+'
+        match('+')
+        value += term
+      when '-'
+        match('-')
+        value -= term
+      end
+    end
+    value
+  end
+
+  # Parse one or more newlines.
+  def newline
+    if @look == "\n" || @look == "\r"
+      get_char while @look == "\n" || @look == "\r"
+    else
+      expected('newline')
+    end
+  end
+
+  # Parse and evaluate an assignment statement.
+  def assignment
+    name = get_name
+    match('=')
+    set(name, expression)
+  end
+
+  # Parse and evaluate any kind of statement.
+  def statement
+    value = case @look
+            when '?': input
+            when '!': output
+            else
+              assignment
+            end
+    newline
+    value
+  end
+
+  # Read and store a number from standard input.
+  def input
+    match('?')
+    set(get_name, gets.to_i)
+  end
+
+  # Print a value to standard output.
+  def output
+    match('!')
+    puts(get(get_name))
+  end
+
+
+private
+
+  def eof?
+    @input.eof? && @look.nil?
+  end
+
+  def addop?
+    @look == '+' || @look == '-'
+  end
+
+  def mulop?
+    @look == '*' || @look == '/'
+  end
+end
diff --git a/testi.code b/testi.code
new file mode 100644
index 0000000..d1bd085
--- /dev/null
+++ b/testi.code
@@ -0,0 +1 @@
+(5*(3 - 5)*2 - 33 - 9/3 - 8/2 + 4*(5 + 5 + 5)) + (4*5 - 21/2 - 15 + 5)
diff --git a/testi.rb b/testi.rb
new file mode 100644
index 0000000..dbd2dc6
--- /dev/null
+++ b/testi.rb
@@ -0,0 +1,25 @@
+require 'interpreter'
+require 'stringio'
+
+def error(msg) STDERR.puts(msg) end
+
+def eval(input)
+  interpreter = Interpreter.new(input)
+  interpreter.run
+
+rescue ParseError => e
+  error("[error] #{e.message}")
+  error("Aborting!")
+  exit(1)
+end
+
+def main(arg)
+  input = if File.readable?(arg)
+            File.open(arg)
+          else
+            STDIN
+          end
+  puts(eval(input))
+end
+
+main(ARGV[0].to_s)