Search and replace regex in large files without getting OutOfMemoryException

Question

I created a little code which searches for a Regex string and replaces that with something else, it then creates a new output file with the changes made. The code seems to work well with smaller files, but for 100 MB or larger files I am giving the System.OutOfMemoryException' error.

Here's my code:

string foldername = Path.Combine(
    Environment.GetFolderPath(Environment.SpecialFolder.Desktop),
    String.Format("FIXED_{0}.tmx",
        Path.GetFileNameWithoutExtension(textBox1.Text)));

string text = File.ReadAllText(textBox1.Text);
text = Regex.Replace(text, @"<seg\b[^>]*>", "<seg>", RegexOptions.Multiline);
text = Regex.Replace(text, @"<seg>
                    </tuv>", "<seg></seg></tuv>", RegexOptions.Multiline);

File.WriteAllText(foldername, text);

Visual studio highlights the string text = File.ReadAllText(textBox1.Text);section. I thought that maybe using File.ReadAllLines would work better, but I was not able to make it work with regex.

Can anybody help me on this? I am newbie to C# and probably my code is not the best one.

Answer 1

I'm afraid you have to implement the replacement by yourself. The following is the sample code which uses state-machine to replace <seg\b[^>*]> with <seg>. The only problem it has, that if the file is ended with <seg attr="", then only <seg will be written to the output.

enum TruncateSegState
{
    Idle,
    TagStart,
    TagStartS,
    TagStartSE,
    TagStartSEG,
    TagSEG
}

static void TruncateSeg(StreamReader input, StreamWriter output)
{
    TruncateSegState state = TruncateSegState.Idle;
    while (!input.EndOfStream)
    {
        char ch = (char)input.Read();
        switch (state)
        {
            case TruncateSegState.Idle:
                if (ch == '<')
                    state = TruncateSegState.TagStart;
                output.Write(ch);
                break;
            case TruncateSegState.TagStart:
                if (ch == 's')
                    state = TruncateSegState.TagStartS;
                else
                    state = TruncateSegState.Idle;
                output.Write(ch);
                break;
            case TruncateSegState.TagStartS:
                if (ch == 'e')
                    state = TruncateSegState.TagStartSE;
                else
                    state = TruncateSegState.Idle;
                output.Write(ch);
                break;
            case TruncateSegState.TagStartSE:
                if (ch == 'g')
                    state = TruncateSegState.TagStartSEG;
                else
                    state = TruncateSegState.Idle;
                output.Write(ch);
                break;
            case TruncateSegState.TagStartSEG:
                if (char.IsWhiteSpace(ch))
                    state = TruncateSegState.TagSEG;
                else
                {
                    state = TruncateSegState.Idle;
                    output.Write(ch);
                }
                break;
            case TruncateSegState.TagSEG:
                if (ch == '>')
                {
                    state = TruncateSegState.Idle;
                    output.Write(ch);
                }
                break;
        }
    }
}

Usage:

using (StreamReader reader = new StreamReader("input.txt"))
using (StreamWriter writer = new StreamWriter("temp.txt"))
    TruncateSeg(reader, writer);

After you generated temp.txt, you use it as input for the next method, which adds missing </seg> tag.

enum ReplaceSegTuvState
{
    Idle,
    InsideSEG
}

static void ReplaceSegTuv(StreamReader input, StreamWriter output)
{
    ReplaceSegTuvState state = ReplaceSegTuvState.Idle;
    StringBuilder segBuffer = new StringBuilder();
    while (!input.EndOfStream)
    {
        char ch = (char)input.Read();
        switch (state)
        {
            case ReplaceSegTuvState.Idle:
                if (ch == '<')
                {
                    char[] buffer = new char[4];
                    int bufferActualLength = input.ReadBlock(buffer, 0, buffer.Length);
                    output.Write('<');
                    output.Write(buffer, 0, bufferActualLength);
                    if (bufferActualLength == buffer.Length && "seg>".SequenceEqual(buffer))
                    {
                        segBuffer.Clear();
                        state = ReplaceSegTuvState.InsideSEG;
                    }
                }
                else
                    output.Write(ch);
                break;
            case ReplaceSegTuvState.InsideSEG:
                if (ch == '<')
                {
                    char[] buffer = new char[5];
                    int bufferActualLength = input.ReadBlock(buffer, 0, buffer.Length);
                    if (bufferActualLength == buffer.Length && "/tuv>".SequenceEqual(buffer))
                    {
                        output.Write("</seg>");
                        output.Write("</tuv>");
                        state = ReplaceSegTuvState.Idle;
                    }
                    else
                    {
                        output.Write(segBuffer.ToString());
                        output.Write('<');
                        output.Write(buffer, 0, bufferActualLength);
                        state = ReplaceSegTuvState.Idle;
                    }
                }
                else if (!char.IsWhiteSpace(ch))
                {
                    output.Write(segBuffer.ToString());
                    output.Write(ch);
                    state = ReplaceSegTuvState.Idle;
                }
                else
                    segBuffer.Append(ch);
                break;
        }
    }
}

Usage:

using (StreamReader reader = new StreamReader("temp.txt"))
using (StreamWriter writer = new StreamWriter("output.txt"))
    ReplaceSegTuv(reader, writer);