using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string pattern = @"(?<=^(?:\w+=)?)\w+(?=;)";
        string input = @"word2;word3
word1=word2;word3";
        RegexOptions options = RegexOptions.Multiline;
        
        foreach (Match m in Regex.Matches(input, pattern, options))
        {
            Console.WriteLine( m.Value);
        }
	}
}