abp/modules/docs/src/Volo.Docs.Web/Pages/Documents/Project/HtmlNormalizer.cs

using System;
using System.Text.RegularExpressions;

namespace Volo.Docs.Pages.Documents.Project
{
    public static class HtmlNormalizer
    {
        public static string ReplaceImageSources(string content, string documentRawRootUrl, string localDirectory)
        {
            content = Regex.Replace(content, @"(<img\s+[^>]*)src=""([^""]*)""([^>]*>)", delegate (Match match)
                {
                    var newImageSource = documentRawRootUrl.EnsureEndsWith('/') +
                                         (localDirectory.IsNullOrEmpty() ? "" : localDirectory.TrimStart('/').EnsureEndsWith('/')) +
                                         match.Groups[2].Value.TrimStart('/');
                    return match.Groups[1] + " src=\"" + newImageSource + "\" " + match.Groups[3];

                }, RegexOptions.IgnoreCase | RegexOptions.Singleline | RegexOptions.Multiline);


            return content;
        }
    }
}