sharpcompress/src/SharpCompress/Archives/Zip/ZipArchiveVolumeFactory.cs

using System;
using System.Collections.Generic;
using System.IO;
using SharpCompress.Common.Rar;
using SharpCompress.Readers;
using System.Linq;
using System.Text;
using SharpCompress.Common.Rar.Headers;
using System.Text.RegularExpressions;

namespace SharpCompress.Archives.Zip
{
    internal static class ZipArchiveVolumeFactory
    {
        internal static FileInfo? GetFilePart(int index, FileInfo part1) //base the name on the first part
        {
            FileInfo? item = null;

            //load files with zip/zipx first. Swapped to end once loaded in ZipArchive
            //new style .zip, z01.. | .zipx, zx01 - if the numbers go beyond 99 then they use 100 ...1000 etc
            Match m = Regex.Match(
                part1.Name,
                @"^(.*\.)(zipx?|zx?[0-9]+)$",
                RegexOptions.IgnoreCase
            );
            if (m.Success)
                item = new FileInfo(
                    Path.Combine(
                        part1.DirectoryName!,
                        String.Concat(
                            m.Groups[1].Value,
                            Regex.Replace(m.Groups[2].Value, @"[^xz]", ""),
                            index.ToString().PadLeft(2, '0')
                        )
                    )
                );
            else //split - 001, 002 ...
                return ArchiveVolumeFactory.GetFilePart(index, part1);

            if (item != null && item.Exists)
                return item;

            return null; //no more items
        }
    }
}