Question

I have a binary file (i.e., it contains bytes with values between 0x00 and 0xFF). There are also ASCII strings in the file (e.g., “Hello World”) that I want to find and edit using Regex. I then need to write out the edited file so that it’s exactly the same as the old one but with my ASCII edits having been performed. How?

        byte[] inbytes = File.ReadAllBytes(wfile);
        string instring = utf8.GetString(inbytes);
        // use Regex to find/replace some text within instring
        byte[] outbytes = utf8.GetBytes(instring);
        File.WriteAllBytes(outfile, outbytes);

Even if I don’t do any edits, the output file is different from the input file. What’s going on, and how can I do what I want?

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EDIT: Ok, I’m trying to use the offered suggestion and am having trouble understanding how to actually implement it. Here’s my sample code:

        string infile = @"C:\temp\in.dat";
        string outfile = @"C:\temp\out.dat";
        Regex re = new Regex(@"H[a-z]+ W[a-z]+");  // looking for "Hello World"
        byte[] inbytes = File.ReadAllBytes(infile);
        string instring = new SoapHexBinary(inbytes).ToString();
        Match match = re.Match(instring);
        if (match.Success)
        {
            // do work on 'instring'
        }
        File.WriteAllBytes(outfile, SoapHexBinary.Parse(instring).Value);

Obviously, I know I’ll not get a match doing it that way, but if I convert my Regex to a string (or whatever), then I can’t use Match, etc. Any ideas? Thanks!

Answer 1

I got it! Check out the code:

        string infile = @"C:\temp\in.dat";
        string outfile = @"C:\temp\out.dat";
        Regex re = new Regex(@"H[a-z]+ W[a-z]+");   // looking for "Hello World"
        string repl =  @"Hi there";

        Encoding ascii = Encoding.ASCII;
        byte[] inbytes = File.ReadAllBytes(infile);
        string instr = ascii.GetString(inbytes);
        Match match = re.Match(instr);
        int beg = 0;
        bool replaced = false;
        List<byte> newbytes = new List<byte>();
        while (match.Success)
        {
            replaced = true;
            for (int i = beg; i < match.Index; i++)
                newbytes.Add(inbytes[i]);
            foreach (char c in repl)
                newbytes.Add(Convert.ToByte(c));
            Match nmatch = match.NextMatch();
            int end = (nmatch.Success) ? nmatch.Index : inbytes.Length;
            for (int i = match.Index + match.Length; i < end; i++)
                newbytes.Add(inbytes[i]);
            beg = end;
            match = nmatch;
        }
        if (replaced)
        {
            var newarr = newbytes.ToArray();
            File.WriteAllBytes(outfile, newarr);
        }
        else
        {
            File.WriteAllBytes(outfile, inbytes);
        }