预搜索基本概念案例分析

            string x = "used:10246 free:20489";

            Regex r = new Regex(@"(?<!used:)\d{4}");
            Console.WriteLine();
            Console.WriteLine("string:{0} \npatten:{1}", x, r);
            //第一次匹配“sed:10246",其中反向负声明匹配为：”sed:1"，后面的数字为匹配返回结果
            //第二次匹配"free:2048"，其中反向负声明匹配为：”free:"，后面的数字为匹配返回结果
            //包含两个匹配的结果：0246,2048
            MatchCollection mc = r.Matches(x);
            foreach (Match m in mc)
            {
                Console.WriteLine("index:{0}, Value:{1}，length:{2}", m.Index, m.Value, m.Length);
            }


            r = new Regex(@"\d{4}(?<!used:)");
            Console.WriteLine();
            Console.WriteLine("string:{0} \npatten:{1}", x, r);
            //该正在表达式可替换为@"\d{4}",因为符合该条件的内容边界点的左侧肯定不是"used:"。
            //包含两个匹配的结果：1024,2048
            mc = r.Matches(x);
            foreach (Match m in mc)
            {
                Console.WriteLine("index:{0}, Value:{1}，length:{2}", m.Index, m.Value, m.Length);
            }


            r = new Regex(@"(?!used:)\d{4}");
            Console.WriteLine();
            Console.WriteLine("string:{0} \npatten:{1}", x, r);
            //r = new Regex(@"\d{4}");
            //该处的表达式与"\d{4}"等价，匹配项左边界的右边为4个数字，肯定不是"used:"。
            //包含两个匹配的结果：1024,2048
            mc = r.Matches(x);
            foreach (Match m in mc)
            {
                Console.WriteLine("index:{0}, Value:{1}，length:{2}", m.Index, m.Value, m.Length);
            }


            r = new Regex(@"\d{4}(?!used:)");
            Console.WriteLine();
            Console.WriteLine("string:{0} \npatten:{1}", x, r);
            //该表达式表示匹配项右边界的右边不能为"used:"，
            //包含两个匹配的结果：1024,2048

            mc = r.Matches(x);
            foreach (Match m in mc)
            {
                Console.WriteLine("index:{0}, Value:{1}，length:{2}", m.Index, m.Value, m.Length);
            }

            x = "aaa ffffff 999999999";
            r = new Regex(@"(\w)((?=\1\1\1)(\1))+");
            Console.WriteLine();
            Console.WriteLine("string:{0} \npatten:{1}", x, r);
            //匹配结果为：ffff,9999999
            mc = r.Matches(x);
            foreach (Match m in mc)
            {
                Console.WriteLine("index:{0}, Value:{1}，length:{2}", m.Index, m.Value, m.Length);
            }