IntersectWithHashset.cs

internal static IEnumerable<T> MyIntersect<T>(this IEnumerable<T> first, IEnumerable<T> second) {
   Debug.Assert(first != null);
   Debug.Assert(second != null);
   var firstHashset = first as HashSet<T>;
   var secondHashset = second as HashSet<T>;
   if (firstHashset != null && secondHashset != null) {
      return (firstHashset.Count > secondHashset.Count)
                ? firstHashset.Intersect(second)
                : secondHashset.Intersect(first);
   }

dperini

//
// Regular Expression for URL validation
//
// Author: Diego Perini
// Created: 2010/12/05
// Updated: 2018/09/12
// License: MIT
//
// Copyright (c) 2010-2018 Diego Perini (http://www.iport.it)
//