First pass at parser

message.treetop

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+grammar Message
+  rule message
+    expression+ {
+      def interpret
+        Regexp.compile(elements.map {|e| e.interpret }.join)
+      end
+    }
+  end
+
+  rule expression
+    string / identifier / optional
+  end
+
+  rule string
+    [^><\[\]]+ {
+      def interpret
+        Regexp.escape(text_value)
+      end
+    }
+  end
+
+  rule identifier
+    '<' identifier_value '>'
+  end
+
+  rule identifier_value
+    [a-zA-Z_] [a-zA-Z_0-9 ]* {
+      def interpret
+        "(.*)"
+      end
+    }
+  end  
+
+  rule optional
+    "[" expression+ "]" {
+      def interpret
+        "(?:" + elements.map {|e| e.interpret }.join + ")?"
+      end
+    }
+  end
+end

test_message_parser.rb

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+require 'treetop'
+require 'polyglot'
+require 'message'
+require 'test/unit'
+
+class Treetop::Runtime::SyntaxNode
+  def interpret
+    elements.map {|e| e.interpret }.join if nonterminal?()
+  end
+end
+
+class TestMessageParser < Test::Unit::TestCase
+  def setup
+    @parser = MessageParser.new
+  end
+
+  def test_string
+    ast = @parser.parse("Hello, world!")
+    assert_equal /Hello,\ world!/, ast.interpret
+  end
+
+  def test_identifier
+    ast = @parser.parse("<id>")
+    assert_equal /(.*)/, ast.interpret
+  end
+
+  def test_optional
+    ast = @parser.parse("[name]")
+    assert_equal /(?:name)?/, ast.interpret
+  end
+
+  def test_composite_expression
+    ast = @parser.parse("hello [[some] <person>]")
+    assert_equal /hello\ (?:(?:some)?\ (.*))?/, ast.interpret
+  end
+
+  def test_strings_are_regexp_escaped
+    ast = @parser.parse("dr. foo")
+    assert_equal /dr\.\ foo/, ast.interpret
+  end
+end