MarkdownPlainText.cs

using System.Text.RegularExpressions;

namespace ShellDocs.Core;

// Prose-only text for search bodies: strips frontmatter, fences, tags/HTML and markdown syntax.
public static class MarkdownPlainText
{
    private static readonly Regex Frontmatter = new(@"^---\s*\r?\n[\s\S]*?\r?\n---\s*\r?\n", RegexOptions.Compiled);
    private static readonly Regex FencedBlock = new(@"^```[\s\S]*?^```", RegexOptions.Multiline | RegexOptions.Compiled);
    private static readonly Regex HtmlTag     = new(@"<[^>]+>", RegexOptions.Compiled);
    private static readonly Regex Image       = new(@"!\[([^\]]*)\]\([^\)]*\)", RegexOptions.Compiled);
    private static readonly Regex Link        = new(@"\[([^\]]+)\]\([^\)]*\)", RegexOptions.Compiled);
    private static readonly Regex InlineCode  = new(@"`([^`]+)`", RegexOptions.Compiled);
    private static readonly Regex Emphasis    = new(@"(\*\*|__|\*|_)(.+?)\1", RegexOptions.Compiled);
    private static readonly Regex HeadingHash = new(@"^#{1,6}\s+", RegexOptions.Multiline | RegexOptions.Compiled);
    private static readonly Regex ListMarker  = new(@"^\s*[-*+]\s+|^\s*\d+\.\s+", RegexOptions.Multiline | RegexOptions.Compiled);
    private static readonly Regex Blockquote  = new(@"^>\s?", RegexOptions.Multiline | RegexOptions.Compiled);
    private static readonly Regex WhitespaceRun = new(@"\s+", RegexOptions.Compiled);

    public static string Extract(string markdown, int maxLength = 8000)
    {
        if (string.IsNullOrEmpty(markdown)) return "";

        var text = Frontmatter.Replace(markdown, "");
        text = FencedBlock.Replace(text, " ");
        text = HtmlTag.Replace(text, " ");
        text = Image.Replace(text, "$1");
        text = Link.Replace(text, "$1");
        text = InlineCode.Replace(text, "$1");
        text = Emphasis.Replace(text, "$2");
        text = HeadingHash.Replace(text, "");
        text = ListMarker.Replace(text, "");
        text = Blockquote.Replace(text, "");
        text = WhitespaceRun.Replace(text, " ").Trim();

        return text.Length > maxLength ? text[..maxLength] : text;
    }
}