Negative Lookahead with Regular expression in Kusto Log Analytics

Hello everyone,

I'm trying to extract exceptions within our logs using regular expression. And it happens that I need to perform a negative lookahead to ignore a specific string. The query looks like below:

let pattern = @'(?!Exception: System.Web.HttpUnhandledException)Exception: (.+)\s+Message: ([\S\s]+)\s+Source: ([\S\s]+)';
let standardize = (msg:string) {
    let msg2 = replace(@'"(\S+)"', '"xxx"', msg);
    replace(@"'(\S+)'", '"xxx"', msg2)
};
WebsiteLogs
| where ExceptionMessage contains "System.Web.HttpUnhandledException" or ExceptionMessage contains "[Extra Exception from config]"
| where  TimeGenerated > ago(7d) 
| project   URL,
            Exception=extract(pattern, 1, ExceptionMessage ), 
            Message = standardize(extract(pattern, 2, ExceptionMessage )),
            StackTrace = substring(extract(pattern, 3, ExceptionMessage ), 0, 500)
| summarize Errors_Count = count() by Message, URL, StackTrace
| sort by Errors_Count desc  nulls last 
| limit 1000

Exception: System.Web.HttpUnhandledException
Message: An unhandled exception occurred.
Source: Sitecore.Mvc
at Sitecore.Mvc.Pipelines.MvcEvents.Exception.ShowAspNetErrorMessage.ShowErrorMessage(ExceptionContext exceptionContext, ExceptionArgs args)
at Sitecore.Mvc.Pipelines.MvcEvents.Exception.ShowAspNetErrorMessage.Process(ExceptionArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
at Sitecore.Mvc.Pipelines.PipelineService.RunPipeline[TArgs](String pipelineName, TArgs args)
at Sitecore.Mvc.Filters.PipelineBasedRequestFilter.OnException(ExceptionContext exceptionContext)
at System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList`1 filters, Exception exception)
at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
at System.Web.Mvc.Controller.ExecuteCore()
at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
at Sitecore.Mvc.Controllers.ControllerRunner.ExecuteController(Controller controller)
at Sitecore.Mvc.Controllers.ControllerRunner.Execute()
at Sitecore.Mvc.Presentation.ControllerRenderer.Render(TextWriter writer)
at Sitecore.Mvc.Pipelines.Response.RenderRendering.ExecuteRenderer.Render(Renderer renderer, TextWriter writer, RenderRenderingArgs args)

Nested Exception

Exception: System.ArgumentNullException
Message: Value cannot be null.
Parameter name: item
Source: Website.Module.Product

Thank you in advance for your precious help