How to exclude part of a string from a regular expression match?

[Sal Sal]

If I have a string as follows

XXXasdf23s5\r\n
asdflkoirfn329i 4\r\n
sef29384ewrj280 39\r\n
XXX123sd3t334\r \n
sdorfu23984rr\r \n
sdflk2893rjf\r\ n
weirj2983jhwer2 398\r\n
XXX12356789\r\n
4w3ujr523q938er \r\n

basically I am searching for sections that begin with XXX but I don't know what the end will be. All I know is that the next section will definitely start with 'XXX'

Can I search for ^XXX.*\r\n(.*\r \n)*XXX

but add something to the regular expression to backtrack to before the next XXX that I matched and ignore it? I want my match to exclude it.

(I tried (?=XXX) but I could not get it to work)

System.Text.RegularExpressions.Regex xxxRegex = new System.Text.RegularExpressions.Regex(@"^XXX\|.*\r\n(.*\r\n)+(?=XXX)", System.Text.RegularExpressions.RegexOptions.Multiline | System.Text.RegularExpressions.RegexOptions.Compiled);

Thanks for any advice

[Sal Sal]

I changed my regular expression to this and it worked

^XXX.*\r\n(.*\r \n)+(?=^XXX)

However sometimes when the match contains multiple XXX sections, it matches all the way to the last one and then my match includes multiple sections. I only want it to match one, any suggestions?

[Samuel Jones]

This is a complicated way of doing it, but in one of fully-fledged programs, it has worked without fail.

This method of doing it will create a LIST of ALL matches, you can then use this list as you wish.

I have added comments to try to explain the process i have used.

Here is the code to do it:

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string original = "XXXasdf23s5\r\nasdflkoirfn329i4\r\nsef29384ewrj28039\r\nXXX123sd3t334\r\nsdorfu23984rr\r\nsdflk2893rjf\r\nweirj2983jhwer2398\r\nXXX12356789\r\n4w3ujr523q938er\r\n";
            
            // Generate list of items
            List<string> items = extractData(original, "XXX");

            // After this line you are left with a list
            // of all the items that were found, to do
            // with as you wish

            // #----- Example -----#
            // Generate Message Box with a list of the
            // found items

            string message = "";
            int itemNo = 0;

            // Display original string
            MessageBox.Show("Orignal String:\r\n\r\n" + original);

            foreach (string item in items)
            {
                message = message + "\r\nItem " + itemNo + ":\r\n" + item;
                itemNo++;
            }
            MessageBox.Show(message);
            // #--- End Example ---#
        }
        
        /// <summary>
        /// This method will extract all items
        /// starting with the tag provided.
        /// </summary>
        /// <param name="contents">This is the string that the items shall be extracted from.</param>
        /// <param name="tag">This is the string that each item is 'tagged' with.</param>
        /// <returns></returns>
        private List<string> extractData(string contents, string tag)
        {
            string item = "";
            int indexStart = 0;
            int indexEnd = 0;
            List<string> itemsList = new List<string>();

            // Will continue to loop until all items are found
            while (indexStart != -1 && indexEnd != -1)
            {
                // Find first occurance
                indexStart = contents.IndexOf(tag);

                // If indexStart = -1, it didn't find a tag,
                // therefore do nothing.

                if (indexStart != -1)
                {
                    // i.e. found an occurance

                    // If tag is found, Trim contents before the tag
                    contents = contents.Remove(0, indexStart + tag.Length);

                    // Find next tag
                    indexEnd = contents.IndexOf(tag);

                    if (indexEnd != -1)
                    {
                        // i.e. found another tag

                        // Isolate item from contents
                        item = contents.Substring(0, indexEnd - 1);

                        // Remove item from contents
                        contents = contents.Remove(0, indexEnd - 1);
                        
                        // Add item to list
                        itemsList.Add(item);
                    }
                    else
                    {
                        // i.e. couldn't find another tag,
                        // therefore it must be the last item

                        // Add the rest of contents to list
                        itemsList.Add(contents);
                    }
                }
            }

            return itemsList;
        }