Returning the matching characters with the result with IndexOf

I have a large text file and looking to find some data

Start1234 …data…End

I need to match on Start that has 4 chars in front of it (1234 is just an example) and four spaces in front of that then read to End. This would be fine using IndexOf returning data but I need to return the 1234 with the result, but it is part of the match so won’t be included. Any ideas on how I can to do this?

> I need to return the 1234 with the result, but it is part of the match
> so won’t be included

That's wrong. If you use text.IndexOf("1234") you will get the index before 1234, so it's included:

string text  = "Start1234 …data…End";
string find = "1234";
int index = text.IndexOf(find);
string result = index == - 1
    ? null
    : text.Substring(index); // 1234 …data…End

Correct me if I'm wrong: you need to find the first instance of the word "Start" which is followed by four non-whitespace characters, then four spaces. You then need to extract those four characters and then everything after the four spaces.

If I am correct in my understanding, then this function should do what you want:

public bool ParseData(string input, out string trailingFour, out string data)
{
	//Regex explanation
	//(Start): matches the word Start
	//(\S{4}): matches any four characters that are not whitespace
	//( {4}): matches four spaces
	//([\s\S]*): matches all remaining chacters in the string, including newlines
	Regex rx = new Regex(@"(Start)(\S{4})( {4})([\s\S]*)");
	
	//A successful match will have 5 capture groups
    var match = rx.Match(input);
	trailingFour = match.Success ? match.Groups[2].Value : string.Empty;
	data = match.Success ? match.Groups[4].Value : string.Empty;
	
	return match.Success;
}

Use RegEx instead of the string methods for this problem. Form a pattern to capture lines:

• Start with literally Start word.
• Followed by any four characters (.{4}). Grouped to get the value.
• Followed by four whitespaces \s{4}.
• Followed by some text (.*?). Grouped to get the value, and
• End with literally End word.

Put it together:

Start(.{4})\s{4}(.*?)End

Example

var input = "Start1234 …data…End Start3453    sdfsdfsdfsEnd\nStartSLDE    some data.End";
var pattern = @"Start(.{4})\s{4}(.*?)End";

foreach (Match m in Regex.Matches(input, pattern))
    Console.WriteLine($"{m.Value}, 1st Group: {m.Groups[1].Value}, 2nd Group: {m.Groups[2].Value}");

This will return two matches only:

Start3453    sdfsdfsdfsEnd, 1st Group: 3453, 2nd Group: sdfsdfsdfs
StartSLDE    some data.End, 1st Group: SLDE, 2nd Group: some data.

Call RegEx.Replace method If you need to replace the matches with something else and return a new string:

var replaced = Regex.Replace(input, pattern, string.Empty);

regex101