diff --git a/ruby/lox.rb b/ruby/lox.rb
index e2b1c4a..e1dde32 100755
--- a/ruby/lox.rb
+++ b/ruby/lox.rb
@@ -1,5 +1,7 @@
 #!/usr/bin/env ruby -w
 
+require "strscan"
+
 module Lox
   class Error < StandardError
     def initialize(line:, where: "", message:)
@@ -32,7 +34,7 @@ module Lox
   end
 
   def self.run(src)
-    Runner.new(src).run
+    Runner.new.run(src)
   end
 
   def self.error(line, msg)
@@ -40,17 +42,118 @@ module Lox
   end
 
   class Runner
-    def initialize(scanner:)
+    def initialize(scanner: Scanner.new)
       @scanner = scanner
     end
 
     def run(src)
-      @scanner.scan(src)
+      @scanner.scan(src).each do |token|
+        puts token
+      end
     end
   end
 
   class Scanner
+    KEYWORDS = {
+      and:    :AND,
+      class:  :CLASS,
+      else:   :ELSE,
+      false:  :FALSE,
+      for:    :FOR,
+      fun:    :FUN,
+      if:     :IF,
+      nil:    :NIL,
+      or:     :OR,
+      print:  :PRINT,
+      return: :RETURN,
+      super:  :SUPER,
+      this:   :THIS,
+      true:   :TRUE,
+      var:    :VAR,
+      while:  :WHILE,
+    }.transform_keys(&:to_s)
+
+    State = Struct.new(:ss, :tokens, :errors, :line) do
+      def eos? = ss.eos?
+      def scan(re) = ss.scan(re)
+      def pos = ss.pos
+
+      def add_token(type, text: nil, literal: nil)
+        text ||= ss.matched
+        self.tokens << Token.new(type, text, literal, line)
+      end
+    end
+
     def scan(src)
+      state = State.new(StringScanner.new(src), [], [], 1)
+
+      until state.eos?
+        case
+        when state.scan(/\(/) then state.add_token(:LEFT_PAREN)
+        when state.scan(/\)/) then state.add_token(:RIGHT_PAREN)
+        when state.scan(/\{/) then state.add_token(:LEFT_BRACE)
+        when state.scan(/}/)  then state.add_token(:RIGHT_BRACE)
+        when state.scan(/,/)  then state.add_token(:COMMA)
+        when state.scan(/\./) then state.add_token(:DOT)
+        when state.scan(/-/)  then state.add_token(:MINUS)
+        when state.scan(/\+/) then state.add_token(:PLUS)
+        when state.scan(/;/)  then state.add_token(:SEMICOLON)
+        when state.scan(/\*/) then state.add_token(:STAR)
+        when state.scan(/!=/) then state.add_token(:BANG_EQUAL)
+        when state.scan(/!/)  then state.add_token(:BANG)
+        when state.scan(/==/) then state.add_token(:EQUAL_EQUAL)
+        when state.scan(/=/)  then state.add_token(:EQUAL)
+        when state.scan(/<=/) then state.add_token(:LESS_EQUAL)
+        when state.scan(/</)  then state.add_token(:LESS)
+        when state.scan(/>=/) then state.add_token(:GREATER_EQUAL)
+        when state.scan(/>/)  then state.add_token(:GREATER)
+        when state.scan(/\/\/(?~\n)+/) # ignore comment
+        when state.scan(/\//) then state.add_token(:SLASH)
+        when state.scan(/[ \r\t]/)    # ignore whitespace
+        when state.scan(/\n/)         then state.line += 1
+        when state.scan(/"/)
+          scan_str(state)
+        when number = state.scan(/\d+(\.\d+)?/)
+          state.add_token(:NUMBER, literal: number.to_f)
+        when identifier = state.scan(/[a-zA-Z_]\w+/)
+          type = KEYWORDS.fetch(identifier, :IDENTIFIER)
+          state.add_token(type)
+        else
+          state.errors << Error.new(line: state.line, message: "Unexpected character.")
+        end
+      end
+
+      state.tokens
+    end
+
+    private
+
+    def scan_str(state)
+      text = ?"
+      loop do
+        case
+        when state.scan(/"/)
+          text << ?"
+          state.add_token(:STRING, text:, literal: text[1..-2])
+          return
+        when state.scan(/\n/)
+          text << ?\n
+          state.line += 1
+        when state.eos?
+          state.errors << Error.new(line: state.line, message: "Unterminated string.")
+          return
+        when c = state.scan(/./)
+          text << c
+        else
+          fail "unreachable!"
+        end
+      end
+    end
+  end
+
+  Token = Struct.new(:type, :lexeme, :literal, :line) do
+    def to_s
+      "#{type} #{lexeme} #{literal}"
     end
   end
 end
diff --git a/ruby/test_lox.rb b/ruby/test_lox.rb
index b0f493a..b9aee89 100644
--- a/ruby/test_lox.rb
+++ b/ruby/test_lox.rb
@@ -31,3 +31,72 @@ class TestRunner < Minitest::Test
     assert_equal %w[ some tokens ], tokens
   end
 end
+
+class TestScanner < Minitest::Test
+  def setup
+    @scanner = Lox::Scanner.new
+  end
+
+  def test_basic_tokens
+    %w[( LEFT_PAREN
+       ) RIGHT_PAREN
+       { LEFT_BRACE
+       } RIGHT_BRACE
+       , COMMA
+       . DOT
+       - MINUS
+       + PLUS
+       ; SEMICOLON
+       * STAR
+       != BANG_EQUAL
+       ! BANG
+       == EQUAL_EQUAL
+       = EQUAL
+       <= LESS_EQUAL
+       < LESS
+       >= GREATER_EQUAL
+       > GREATER
+       / SLASH].each_slice(2).to_h.transform_values(&:to_sym).each do |str, token_type|
+         assert_equal [token_type.to_sym], @scanner.scan(str).map(&:type)
+       end
+  end
+
+  def test_comments_and_whitespace
+    tokens = @scanner.scan(<<~SRC)
+      (\t) // here lies a comment
+      .
+    SRC
+
+    assert_equal %i[LEFT_PAREN RIGHT_PAREN DOT], tokens.map(&:type)
+  end
+
+  def test_line_numbers
+    tokens = @scanner.scan(<<~SRC)
+      (
+      )
+    SRC
+
+    assert_equal [1, 2], tokens.map(&:line)
+  end
+
+  def test_strings
+    assert_equal [Lox::Token.new(:STRING, '""', "", 1)], @scanner.scan('""')
+    assert_equal [], @scanner.scan('"') # TODO test the error once it's exposed
+    assert_equal [Lox::Token.new(:STRING, '"foo"', "foo", 1)], @scanner.scan('"foo"')
+    assert_equal [Lox::Token.new(:STRING, "\"foo\nbar\"", "foo\nbar", 2)], @scanner.scan("\"foo\nbar\"")
+  end
+
+  def test_numbers
+    assert_equal [
+      Lox::Token.new(:NUMBER, "123", 123.0, 1),
+      Lox::Token.new(:NUMBER, "123.4", 123.4, 1),
+    ], @scanner.scan("123 123.4")
+  end
+
+  def test_identifiers
+    assert_equal [
+      Lox::Token.new(:OR, "or", nil, 1),
+      Lox::Token.new(:IDENTIFIER, "orchid", nil, 1),
+    ], @scanner.scan("or orchid")
+  end
+end