$parse

<div ng-app="MyApp">
    <div ng-controller="MyController">
        <input type="text" ng-model="expression" />
        <div>{{ParsedValue}}</div>
    </div>
</div>

angular.module("MyApp",[])
.controller("MyController", function($scope, $parse){
    $scope.$watch("expression", function(newValue, oldValue, context){
        if(newValue !== oldValue){
            var parseFunc = $parse(newValue);
            $scope.ParsedValue = parseFunc(context);
        }
    });
});