using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = "5/29/17;08:17;Fulano;Teatro Companhia\nRua do Canto, 301/305\nEsquina da rua meio\nDe segunda a sexta das 9h as 18h\n5/29/17;08:18;Ciclano;legal";

        var semLookahead = Regex.Replace(input, "\n[^0-9]", "|");

        Console.WriteLine(semLookahead);

​

        Console.WriteLine();        

​

        var comLookahead = Regex.Replace(input, "\n(?=[^0-9])", "|");

        Console.WriteLine(comLookahead);

    }

}