Apply a Regex on Stream?

Question

I\'m searching for fast and safe way to apply Regular Expressions on Streams.

I found some examples over the internet that talking about converting each buffer to St

Answer 1

You could add an extra method to StreamReader (the source code of e.g. Mono could be used for that purpose):

    private StringBuilder lineBuilder;
    public int RegexBufferSize
    {
        set { lastRegexMatchedLength = value; }
        get { return lastRegexMatchedLength; }
    }
    private int lastRegexMatchedLength = 0;

    public virtual string ReadRegex(Regex regex)
    {
        if (base_stream == null)
            throw new ObjectDisposedException("StreamReader", "Cannot read from a closed RegexStreamReader");

        if (pos >= decoded_count && ReadBuffer() == 0)
            return null; // EOF Reached

        if (lineBuilder == null)
            lineBuilder = new StringBuilder();
        else
            lineBuilder.Length = 0;

        lineBuilder.Append(decoded_buffer, pos, decoded_count - pos);
        int bytesRead = ReadBuffer();

        bool dataTested = false;
        while (bytesRead > 0)
        {
            var lineBuilderStartLen = lineBuilder.Length;
            dataTested = false;
            lineBuilder.Append(decoded_buffer, 0, bytesRead);

            if (lineBuilder.Length >= lastRegexMatchedLength)
            {
                var currentBuf = lineBuilder.ToString();
                var match = regex.Match(currentBuf, 0, currentBuf.Length);
                if (match.Success)
                {
                    var offset = match.Index + match.Length;
                    pos = 0;
                    decoded_count = lineBuilder.Length - offset;
                    ensureMinDecodedBufLen(decoded_count);
                    lineBuilder.CopyTo(offset, decoded_buffer, 0, decoded_count);
                    var matchedString = currentBuf.Substring(match.Index, match.Length);
                    return matchedString;
                }
                else
                {
                    lastRegexMatchedLength *= (int) 1.1; // allow for more space before attempting to match
                    dataTested = true;
                }
            }

            bytesRead = ReadBuffer();
        }

        // EOF reached

        if (!dataTested)
        {
            var currentBuf = lineBuilder.ToString();
            var match = regex.Match(currentBuf, 0, currentBuf.Length);
            if (match.Success)
            {
                var offset = match.Index + match.Length;
                pos = 0;
                decoded_count = lineBuilder.Length - offset;
                ensureMinDecodedBufLen(decoded_count);
                lineBuilder.CopyTo(offset, decoded_buffer, 0, decoded_count);
                var matchedString = currentBuf.Substring(match.Index, match.Length);
                return matchedString;

            }
        }
        pos = decoded_count;

        return null;
    }

In the above method, the following vars are used:

1. decoded_buffer : the char buffer that contains/will contain the data read
2. pos: offset within the array containing unhandled data
3. decoded_count: the last element within the buffer containing read data
4. RegexBufferSize: the minimum size of the regex input before any matching occurs.

The method ReadBuffer() needs to read data from the stream. The method ensureMinDecodedBufLen() needs to make sure that the decoded_buffer is large enough.

When calling the method, pass the Regex that needs to be matched against.

Answer 2

Intel has recently open sourced hyperscan library under BSD license. It's a high-performance non-backtracking NFA-based regex engine.

Features: ability to work on streams of input data and simultaneous multiple patterns matching. The last one differs from (pattern1|pattern2|...) approach, it actually matches patterns concurrently.

It also utilizes Intel's SIMD instructions sets like SSE4.2, AVX2 and BMI. The summary of the design and explanation of work can be found here. It also has great developer's reference guide with a lot of explanations as well as performance and usage considerations. Small article about using it in the wild (in russian).

Answer 3

It seems that you would know the start and end delimiters of the matches you are trying to get, correct? (i.e. [,] or START,END etc.) So would it make sense to search for these delimiters as data from your stream comes in and then creating a sub-string between the delimiters and do further processing on those?

I know it's pretty much the same thing as rolling your own, but it will be with a more specific purpose and even be able to process it as it comes in.

The problem with regular expressions in this instance is that they work based on matches so you can only match against the amount of input you have. If you have a stream, you would have to read in all the data to get all the matches (space / time constraint issue), try to match against the character at a time brought in (pretty useless), match in chunks (again, something can be easily missed there) or generate strings of interest which if they match your criteria can be shipped off elsewhere for further processing.