DiscordChatExporter/DiscordChatExporter.Core.Markdown/MarkdownParser.cs

using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using DiscordChatExporter.Core.Markdown.Ast;
using DiscordChatExporter.Core.Markdown.Internal;

namespace DiscordChatExporter.Core.Markdown
{
    // The following parsing logic is meant to replicate Discord's markdown grammar as close as possible
    public static class MarkdownParser
    {
        private const RegexOptions DefaultRegexOptions = RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.Multiline;

        /* Formatting */

        // Capture any character until the earliest double asterisk not followed by an asterisk
        private static readonly IMatcher<Node> BoldFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\*\\*(.+?)\\*\\*(?!\\*)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Bold, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the earliest single asterisk not preceded or followed by an asterisk
        // Opening asterisk must not be followed by whitespace
        // Closing asterisk must not be preceded by whitespace
        private static readonly IMatcher<Node> ItalicFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\*(?!\\s)(.+?)(?<!\\s|\\*)\\*(?!\\*)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Italic, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the earliest triple asterisk not followed by an asterisk
        private static readonly IMatcher<Node> ItalicBoldFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\*(\\*\\*.+?\\*\\*)\\*(?!\\*)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Italic, Parse(p.Slice(m.Groups[1]), BoldFormattedNodeMatcher)));

        // Capture any character except underscore until an underscore
        // Closing underscore must not be followed by a word character
        private static readonly IMatcher<Node> ItalicAltFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("_([^_]+)_(?!\\w)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Italic, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the earliest double underscore not followed by an underscore
        private static readonly IMatcher<Node> UnderlineFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("__(.+?)__(?!_)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Underline, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the earliest triple underscore not followed by an underscore
        private static readonly IMatcher<Node> ItalicUnderlineFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("_(__.+?__)_(?!_)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Italic, Parse(p.Slice(m.Groups[1]), UnderlineFormattedNodeMatcher)));

        // Capture any character until the earliest double tilde
        private static readonly IMatcher<Node> StrikethroughFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("~~(.+?)~~", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Strikethrough, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the earliest double pipe
        private static readonly IMatcher<Node> SpoilerFormattedNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\|\\|(.+?)\\|\\|", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Spoiler, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the end of the line
        // Opening 'greater than' character must be followed by whitespace
        private static readonly IMatcher<Node> SingleLineQuoteNodeMatcher = new RegexMatcher<Node>(
            new Regex("^>\\s(.+)\r?\n?", DefaultRegexOptions),
            (p, m) => new FormattedNode(TextFormatting.Quote, Parse(p.Slice(m.Groups[1]))));

        // Capture any character until the end of the input
        // Opening 'greater than' characters must be followed by whitespace
        private static readonly IMatcher<Node> MultiLineQuoteNodeMatcher = new RegexMatcher<Node>(
            new Regex("^>>>\\s(.+)", DefaultRegexOptions | RegexOptions.Singleline),
            (p, m) => new FormattedNode(TextFormatting.Quote, Parse(p.Slice(m.Groups[1]))));

        /* Code blocks */

        // Capture any character except backtick until a backtick
        // Blank lines at the beginning and end of content are trimmed
        private static readonly IMatcher<Node> InlineCodeBlockNodeMatcher = new RegexMatcher<Node>(
            new Regex("`([^`]+)`", DefaultRegexOptions | RegexOptions.Singleline),
            m => new InlineCodeBlockNode(m.Groups[1].Value.Trim('\r', '\n')));

        // Capture language identifier and then any character until the earliest triple backtick
        // Language identifier is one word immediately after opening backticks, followed immediately by newline
        // Blank lines at the beginning and end of content are trimmed
        private static readonly IMatcher<Node> MultiLineCodeBlockNodeMatcher = new RegexMatcher<Node>(
            new Regex("```(?:(\\w*)\\n)?(.+?)```", DefaultRegexOptions | RegexOptions.Singleline),
            m => new MultiLineCodeBlockNode(m.Groups[1].Value, m.Groups[2].Value.Trim('\r', '\n')));

        /* Mentions */

        // Capture @everyone
        private static readonly IMatcher<Node> EveryoneMentionNodeMatcher = new StringMatcher<Node>(
            "@everyone",
            p => new MentionNode("everyone", MentionType.Meta));

        // Capture @here
        private static readonly IMatcher<Node> HereMentionNodeMatcher = new StringMatcher<Node>(
            "@here",
            p => new MentionNode("here", MentionType.Meta));

        // Capture <@123456> or <@!123456>
        private static readonly IMatcher<Node> UserMentionNodeMatcher = new RegexMatcher<Node>(
            new Regex("<@!?(\\d+)>", DefaultRegexOptions),
            m => new MentionNode(m.Groups[1].Value, MentionType.User));

        // Capture <#123456>
        private static readonly IMatcher<Node> ChannelMentionNodeMatcher = new RegexMatcher<Node>(
            new Regex("<#(\\d+)>", DefaultRegexOptions),
            m => new MentionNode(m.Groups[1].Value, MentionType.Channel));

        // Capture <@&123456>
        private static readonly IMatcher<Node> RoleMentionNodeMatcher = new RegexMatcher<Node>(
            new Regex("<@&(\\d+)>", DefaultRegexOptions),
            m => new MentionNode(m.Groups[1].Value, MentionType.Role));

        /* Emojis */

        // Capture any country flag emoji (two regional indicator surrogate pairs)
        // ... or "miscellaneous symbol" character
        // ... or surrogate pair
        // ... or digit followed by enclosing mark
        // (this does not match all emojis in Discord but it's reasonably accurate enough)
        private static readonly IMatcher<Node> StandardEmojiNodeMatcher = new RegexMatcher<Node>(
            new Regex("((?:[\\uD83C][\\uDDE6-\\uDDFF]){2}|[\\u2600-\\u26FF]|\\p{Cs}{2}|\\d\\p{Me})", DefaultRegexOptions),
            m => new EmojiNode(m.Groups[1].Value));

        // Capture <:lul:123456> or <a:lul:123456>
        private static readonly IMatcher<Node> CustomEmojiNodeMatcher = new RegexMatcher<Node>(
            new Regex("<(a)?:(.+?):(\\d+?)>", DefaultRegexOptions),
            m => new EmojiNode(m.Groups[3].Value, m.Groups[2].Value, !string.IsNullOrWhiteSpace(m.Groups[1].Value)));

        /* Links */

        // Capture [title](link)
        private static readonly IMatcher<Node> TitledLinkNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\[(.+?)\\]\\((.+?)\\)", DefaultRegexOptions),
            m => new LinkNode(m.Groups[2].Value, m.Groups[1].Value));

        // Capture any non-whitespace character after http:// or https:// until the last punctuation character or whitespace
        private static readonly IMatcher<Node> AutoLinkNodeMatcher = new RegexMatcher<Node>(
            new Regex("(https?://\\S*[^\\.,:;\"\'\\s])", DefaultRegexOptions),
            m => new LinkNode(m.Groups[1].Value));

        // Same as auto link but also surrounded by angular brackets
        private static readonly IMatcher<Node> HiddenLinkNodeMatcher = new RegexMatcher<Node>(
            new Regex("<(https?://\\S*[^\\.,:;\"\'\\s])>", DefaultRegexOptions),
            m => new LinkNode(m.Groups[1].Value));

        /* Text */

        // Capture the shrug emoticon
        // This escapes it from matching for formatting
        private static readonly IMatcher<Node> ShrugTextNodeMatcher = new StringMatcher<Node>(
            @"¯\_(ツ)_/¯",
            p => new TextNode(p.ToString()));

        // Capture some specific emojis that don't get rendered
        // This escapes it from matching for emoji
        private static readonly IMatcher<Node> IgnoredEmojiTextNodeMatcher = new RegexMatcher<Node>(
            new Regex("(\\u26A7|\\u2640|\\u2642|\\u2695|\\u267E|\\u00A9|\\u00AE|\\u2122)", DefaultRegexOptions),
            m => new TextNode(m.Groups[1].Value));

        // Capture any "symbol/other" character or surrogate pair preceded by a backslash
        // This escapes it from matching for emoji
        private static readonly IMatcher<Node> EscapedSymbolTextNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\\\(\\p{So}|\\p{Cs}{2})", DefaultRegexOptions),
            m => new TextNode(m.Groups[1].Value));

        // Capture any non-whitespace, non latin alphanumeric character preceded by a backslash
        // This escapes it from matching for formatting or other tokens
        private static readonly IMatcher<Node> EscapedCharacterTextNodeMatcher = new RegexMatcher<Node>(
            new Regex("\\\\([^a-zA-Z0-9\\s])", DefaultRegexOptions),
            m => new TextNode(m.Groups[1].Value));

        // Combine all matchers into one
        // Matchers that have similar patterns are ordered from most specific to least specific
        private static readonly IMatcher<Node> AggregateNodeMatcher = new AggregateMatcher<Node>(
            // Escaped text
            ShrugTextNodeMatcher,
            IgnoredEmojiTextNodeMatcher,
            EscapedSymbolTextNodeMatcher,
            EscapedCharacterTextNodeMatcher,

            // Formatting
            ItalicBoldFormattedNodeMatcher,
            ItalicUnderlineFormattedNodeMatcher,
            BoldFormattedNodeMatcher,
            ItalicFormattedNodeMatcher,
            UnderlineFormattedNodeMatcher,
            ItalicAltFormattedNodeMatcher,
            StrikethroughFormattedNodeMatcher,
            SpoilerFormattedNodeMatcher,
            MultiLineQuoteNodeMatcher,
            SingleLineQuoteNodeMatcher,

            // Code blocks
            MultiLineCodeBlockNodeMatcher,
            InlineCodeBlockNodeMatcher,

            // Mentions
            EveryoneMentionNodeMatcher,
            HereMentionNodeMatcher,
            UserMentionNodeMatcher,
            ChannelMentionNodeMatcher,
            RoleMentionNodeMatcher,

            // Links
            TitledLinkNodeMatcher,
            AutoLinkNodeMatcher,
            HiddenLinkNodeMatcher,

            // Emoji
            StandardEmojiNodeMatcher,
            CustomEmojiNodeMatcher);

        // Minimal set of matchers for non-multimedia formats (e.g. plain text)
        private static readonly IMatcher<Node> MinimalAggregateNodeMatcher = new AggregateMatcher<Node>(
            // Mentions
            EveryoneMentionNodeMatcher,
            HereMentionNodeMatcher,
            UserMentionNodeMatcher,
            ChannelMentionNodeMatcher,
            RoleMentionNodeMatcher,

            // Emoji
            CustomEmojiNodeMatcher);

        private static IReadOnlyList<Node> Parse(StringPart stringPart, IMatcher<Node> matcher) =>
            matcher.MatchAll(stringPart, p => new TextNode(p.ToString())).Select(r => r.Value).ToArray();

        private static IReadOnlyList<Node> Parse(StringPart stringPart) => Parse(stringPart, AggregateNodeMatcher);

        private static IReadOnlyList<Node> ParseMinimal(StringPart stringPart) => Parse(stringPart, MinimalAggregateNodeMatcher);

        public static IReadOnlyList<Node> Parse(string input) => Parse(new StringPart(input));

        public static IReadOnlyList<Node> ParseMinimal(string input) => ParseMinimal(new StringPart(input));
    }
}