text: var types = require('./types');
  exports.wordBoundary = function () {
      return { type: types.POSITION, value: 'b' };
  };
  exports.nonWordBoundary = function () {
      return { type: types.POSITION, value: 'B' };
  };
  exports.begin = function () {
      return { type: types.POSITION, value: '^' };
  };
  exports.end = function () {
      return { type: types.POSITION, value: '$' };
  };
  