using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string[] strings = {
            "...somethingashisomething...",
            "...something2!ashi*&something... ",
            "... something ashi something flashing...",
            "...somethingflashingsomething...",
            "...smashingthesomething...",
            "...the lashings are too tight...   "
            };

        string pattern = @"(?<!(?:sm|f?l)(?=ashing))ashi";
        foreach (String s in strings) {
            if (Regex.IsMatch(s, pattern)) {
                Console.WriteLine("Match: {0}", s);
            } else {
                Console.WriteLine("No match: {0}", s);
            }
        }
	}
}