diff --git a/lib/imbecile.rb b/lib/imbecile.rb
index 05a4b32..5a7e113 100644
--- a/lib/imbecile.rb
+++ b/lib/imbecile.rb
@@ -1,5 +1,6 @@
 require_relative "imbecile/cli"
 require_relative "imbecile/grammar"
+require_relative "imbecile/regex"
 require_relative "imbecile/version"
 require "erb"
 
diff --git a/lib/imbecile/grammar.rb b/lib/imbecile/grammar.rb
index bb5952e..b71823e 100644
--- a/lib/imbecile/grammar.rb
+++ b/lib/imbecile/grammar.rb
@@ -23,6 +23,9 @@ module Imbecile
           @classname = $1
         elsif line =~ /^\s*token\s+(\S+)\s+(.*)$/
           name, expr = $1, $2
+          if expr == ""
+            expr = name
+          end
           unless name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
             raise Error.new("Invalid token name #{name} on line #{line_number}")
           end
@@ -34,6 +37,11 @@ module Imbecile
           raise Error.new("Unexpected input on line #{line_number}: #{line}")
         end
       end
+
+      # Build NFA from each token expression.
+      @tokens.transform_values! do |expr|
+        LexerNFA.new(expr)
+      end
     end
 
   end
diff --git a/lib/imbecile/regex.rb b/lib/imbecile/regex.rb
new file mode 100644
index 0000000..e46fe88
--- /dev/null
+++ b/lib/imbecile/regex.rb
@@ -0,0 +1,218 @@
+module Imbecile
+  class Regex
+
+    class State
+      attr_accessor :accepting
+
+      def initialize
+        @transitions = []
+      end
+
+      def add_transition(character_range, state)
+        @transitions << [character_range, state]
+      end
+    end
+
+    def initialize(pattern)
+      @pattern = pattern.dup
+      parse
+    end
+
+    private
+
+    class Unit
+    end
+
+    class SequenceUnit < Unit
+      attr_accessor :units
+      def initialize
+        @units = []
+      end
+      def method_missing(*args)
+        @units.__send__(*args)
+      end
+    end
+
+    class AlternatesUnit < Unit
+      attr_accessor :alternates
+      attr_accessor :negate
+      def initialize
+        @alternates = []
+        @negate = false
+      end
+      def new_alternate!
+        @alternates << SequenceUnit.new
+      end
+      def append_alternate(unit)
+        @alternates << unit
+      end
+      def <<(unit)
+        new_alternate! if @alternates.empty?
+        @alternates[-1] << unit
+      end
+      def last_unit
+        new_alternate! if @alternates.empty?
+        @alternates[-1][-1]
+      end
+      def replace_last!(new_unit)
+        @alternates[-1][-1] = new_unit
+      end
+    end
+
+    class CharacterUnit < Unit
+      attr_accessor :code_point
+      def initialize(c)
+        @code_point = c.ord
+      end
+    end
+
+    class CharacterRangeUnit < Unit
+      attr_accessor :start_code_point
+      attr_accessor :end_code_point
+      def initialize(c1, c2)
+        @start_code_point = c1.ord
+        @end_code_point = c2.ord
+      end
+    end
+
+    class MultiplicityUnit < Unit
+      attr_accessor :unit
+      attr_accessor :min_count
+      attr_accessor :max_count
+      def initialize(unit, min_count, max_count)
+        @unit = unit
+        @min_count = min_count
+        @max_count = max_count
+      end
+    end
+
+    def parse
+      @unit = parse_alternates
+      if @pattern != ""
+        raise "Invalid pattern: #{@pattern}"
+      end
+    end
+
+    def parse_alternates
+      au = AlternatesUnit.new
+      while @pattern != ""
+        c = @pattern[0]
+        return au if c == ")"
+        @pattern.slice!(0)
+        case c
+        when "["
+          au << parse_character_class
+        when "("
+          au << parse_group
+        when "*", "+", "?", "{"
+          if last_unit = au.last_unit
+            case c
+            when "*"
+              min_count, max_count = 0, nil
+            when "+"
+              min_count, max_count = 1, nil
+            when "?"
+              min_count, max_count = 0, 1
+            when "{"
+              min_count, max_count = parse_curly_count
+            end
+            mu = MultiplicityUnit.new(last_unit, min_count, max_count)
+            au.replace_last!(mu)
+          else
+            raise Error.new("#{c} follows nothing")
+          end
+        when "|"
+          au.new_alternate!
+        when "\\"
+          au << parse_backslash
+        else
+          au << CharacterUnit.new(c)
+        end
+      end
+      au
+    end
+
+    def parse_group
+      au = parse_alternates
+      if @pattern[0] != ")"
+        raise Error.new("Unterminated group in pattern")
+      end
+      @pattern.slice!(0)
+      au
+    end
+
+    def parse_character_class
+      au = AlternatesUnit.new
+      index = 0
+      loop do
+        if @pattern == ""
+          raise Error.new("Unterminated character class")
+        end
+        c = @pattern.slice!(0)
+        if c == "]"
+          break
+        elsif c == "^" && index == 0
+          au.negate = true
+        elsif c == "-" && (index == 0 || @pattern[0] == "]")
+          au.append_alternate(CharacterUnit.new(c))
+        elsif c == "\\"
+          au.append_alternate(parse_backslash)
+        elsif c == "-" && @pattern[0] != "]"
+          begin_cu = au.last_unit
+          unless begin_cu.is_a?(CharacterUnit)
+            raise Error.new("Character range must be between single characters")
+          end
+          if @pattern[0] == "\\"
+            @pattern.slice!(0)
+            end_cu = parse_backslash
+            unless end_cu.is_a?(CharacterUnit)
+              raise Error.new("Character range must be between single characters")
+            end
+            end_code_point = end_cu.code_point
+          else
+            end_code_point = @pattern[0].ord
+            @pattern.slice!(0)
+          end
+          cru = CharacterRangeUnit.new(begin_cu.code_point, end_code_point)
+          au.replace_last!(cru)
+        else
+          au.append_alternate(CharacterUnit.new(c))
+        end
+        index += 1
+      end
+      au
+    end
+
+    def parse_curly_count
+      if @pattern =~ /^(\d+)(?:,(\d+))?\}(.*)$/
+        min_count = $1.to_i
+        max_count = nil
+        if $2 != ""
+          max_count = $2.to_i
+          if max_count < min_count
+            raise Error.new("Maximum repetition count cannot be less than minimum repetition count")
+          end
+        end
+        @pattern = $3
+        [min_count, max_count]
+      else
+        raise Error.new("Unexpected match count at #{@pattern}")
+      end
+    end
+
+    def parse_backslash
+      if @pattern == ""
+        raise Error.new("Error: unfollowed \\")
+      else
+        c = @pattern.slice!(0)
+        case c
+        when "d"
+          CharacterRangeUnit.new("0", "9")
+        else
+          CharacterUnit.new(c)
+        end
+      end
+    end
+
+  end
+end