C#

string fileContent = File.ReadAllText(@"D:\Temp\AATest.txt");
string[] bits = fileContent.Split(new string[] {"ACC_ID:"}, StringSplitOptions.RemoveEmptyEntries);
foreach (string s in bits)
    {
    Console.WriteLine("++++");
    Console.WriteLine(s);
    }

Quote:

When you use ReadLines, you can start enumerating the collection of strings before the whole collection is returned; when you use ReadAllLines, you must wait for the whole array of strings be returned before you can access the array. Therefore, when you are working with very large files, ReadLines can be more efficient.

You can use the ReadLines method to do the following:

Perform LINQ to Objects queries on a file to obtain a filtered set of its lines.

static List<AccountData> ParseAccountData(string filePath, string delimiter = "ACC_ID:")
{
	int g = 0;
	return File.ReadAllLines(filePath)
		.Select(x => new 
		{ 
			group = x == delimiter ? ++g : g,
			line = x 
		})
		.Where(x => x.line != delimiter)
		.GroupBy(x => x.group)
		.Select(x => new AccountData 
		{ 
			Acc_ID = x.First().line,
			Data = x.Skip(1).Select(z => z.line).ToList()
		})
		.ToList()
}

class AccountData
{
	public string Acc_ID { get; set; }
	
	public List<string> Data { get; set; }
}