Can I programmatically apply Angular validation directives inside a custom directive?

One way to do this (i.e. apply existing validators without writing their code again) would be to add the validation directives’ respective attributes and force a re-compile. This would require your directive to have a high-enough priority and also be terminal: true.

app.directive("bkNgValidation", function($compile){
  return {
    priority: 10000,
    terminal: true,
    link: function(scope, element){
      element.attr("ng-required", "true");
      element.attr("ng-minlength", 20);
      element.attr("ng-maxlength", 30);

      // prevent infinite loop
      element.removeAttr("bk-ng-validation");

      $compile(element)(scope);
    }
  };
});

Demo