Optional pattern.

tests/test_textparser.py

@@ -15,6 +15,7 @@
 from textparser import Any
 from textparser import Inline
 from textparser import Forward
+from textparser import Optional
 
 
 def tokenize(items):
@@ -301,6 +302,38 @@ def test_forward(self):
             tree = grammar.parse(tokenize(tokens + [('__EOF__', '')]))
             self.assertEqual(tree, expected_tree)
 
+    def test_optional(self):
+        grammar = Grammar(Sequence(Optional('WORD'),
+                                   Optional('WORD'),
+                                   Optional('NUMBER')))
+
+        datas = [
+            (
+                [],
+                [[], [], []]
+            ),
+            (
+                [('WORD', 'a')],
+                [['a'], [], []]
+            ),
+            (
+                [('NUMBER', 'c')],
+                [[], [], ['c']]
+            ),
+            (
+                [('WORD', 'a'), ('NUMBER', 'c')],
+                [['a'], [], ['c']]
+            ),
+            (
+                [('WORD', 'a'), ('WORD', 'b'), ('NUMBER', 'c')],
+                [['a'], ['b'], ['c']]
+            )
+        ]
+
+        for tokens, expected_tree in datas:
+            tree = grammar.parse(tokenize(tokens + [('__EOF__', '')]))
+            self.assertEqual(tree, expected_tree)
+
 
 if __name__ == '__main__':
     unittest.main()

textparser.py

@@ -280,6 +280,23 @@ def match(self, tokens):
         return self._element.match(tokens)
 
 
+class Optional(object):
+    """Matches a pattern zero or one times.
+
+    """
+
+    def __init__(self, element):
+        self._element = _wrap_string(element)
+
+    def match(self, tokens):
+        mo = self._element.match(tokens)
+
+        if mo is None:
+            return []
+        else:
+            return [mo]
+
+
 class Forward(object):
 
     def __init__(self):