using System;

using System.Linq;

using System.Text.RegularExpressions;

using System.Globalization;

using System.Collections.Generic;

public class Program

{

    private static readonly Regex regex = new Regex(@"^(?<num>\d+)\.", RegexOptions.Compiled);

    public static void Main()

    {

        var list = new [] {

            "1.one",

            "2. two",

            "no number",

            "2.duplicate",

            "300. three hundred",

            "4-ignore this"

        };

​

        var comparer = new GenericCompare<string>(s => regex.Match(s).Groups["num"].Value);

        var distinctWithNumbers = list.Where(s => regex.IsMatch(s)).Distinct(comparer);     

        distinctWithNumbers.ToList().ForEach(Console.WriteLine);

        //Console.ReadKey();

    }

}

​

public class GenericCompare<T> : IEqualityComparer<T> where T : class

{

    private Func<T, object> _expr { get; set; }

    public GenericCompare(Func<T, object> expr)

    {

        this._expr = expr;

    }

    public bool Equals(T x, T y)

    {       

        var first = _expr.Invoke(x);                

        var sec = _expr.Invoke(y);  

        if (first != null && first.Equals(sec))

            return true;

        else

            return false;

    }

    public int GetHashCode(T obj)

    {

        return _expr.Invoke(obj).GetHashCode();

    }

}