using System;

using System.IO;

using System.Linq;

using System.Text;

public class Program

{

    enum ParserState {

        PossibleDriveLetter,

        PossibleDriveLetterSeparator,

        Path

    }

    static string SanitizeFileName(string input) {

        StringBuilder output = new StringBuilder(input.Length);

        ParserState state = ParserState.PossibleDriveLetter;

        foreach(char current in input) {

            if (((current >= 'a') && (current <= 'z')) || ((current >= 'A') && (current <= 'Z'))) {

                output.Append(current);

                if (state == ParserState.PossibleDriveLetter) {

                    state = ParserState.PossibleDriveLetterSeparator;

                }

                else {

                    state = ParserState.Path;

                }

            }

            else if ((current == Path.DirectorySeparatorChar) || (current == Path.AltDirectorySeparatorChar) ||

                ((current == ':') && (state == ParserState.PossibleDriveLetterSeparator)) ||

                !Path.GetInvalidFileNameChars().Contains(current)) {

​

                output.Append(current);

                state = ParserState.Path;

            }

            else {

                output.Append('_');

                state = ParserState.Path;

            }

        }

        return output.ToString();

    }

    public static void Main()

    {

        Console.WriteLine(SanitizeFileName(@"C:\path\something\output_at_13:26:43.txt"));

    }

}