new_contents: import pytest
  from katana.expr import Expr, Token
  from katana.parser import Scanner
  
  
  class TestScanner:
      scanner = Scanner([
          Expr('dollar', r'\$'),
          Expr('number', r'[0-9]+'),
      ])
  
      def test_scan_complete(self):
          assert self.scanner.scan('$12') == [
              Token('dollar', '$'),
              Token('number', '12'),
          ]
  
      def test_scan_incomplete(self):
          with pytest.raises(ValueError):
              self.scanner.scan('12#')
  
subject: Add tests for Scanner class