using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var s = @"_if_111_else_222_if__if_333_else_444_endif__else_555_if_666_else_777_endif__endif__endif_";
		var pat = @"(?x)(?=        # Start of the overlapping match capturing lookahead
			 (_if_                 # Leading delimiter
			  (?>                  # Start of atomic group (no backtracking into it)
			   (?!_(?:end)?if_).   # Any symbol not starting the delimiter sequence 
			   |(?<o>_if_)         # A leading delimiter added to stack o
			   |(?<-o>_endif_)     # Trailing delimiter added to stack o
			  )*                   # Repeat the atomic group 0+ times
			  (?(o)(?!))           # If the o stack is not empty, fail the match
			 _endif_               # Trailing delimiter
			 )
			)";
    	var res = Regex.Matches(s, pat)
    		.Cast<Match>()
    		.Select(p => p.Groups[1].Value)
    		.Where(n => n.Contains("_else_"))
    		.ToList();
        foreach (var v in res)
        	Console.WriteLine(v);
	}
}

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