Discord.Net/src/Discord.Net.Interactions/Utilities/RegexUtils.cs

using System;
using System.Linq;

namespace System.Text.RegularExpressions
{
    internal static class RegexUtils
    {
        internal const byte Q = 5;    // quantifier
        internal const byte S = 4;    // ordinary stoppper
        internal const byte Z = 3;    // ScanBlank stopper
        internal const byte X = 2;    // whitespace
        internal const byte E = 1;    // should be escaped

        internal static readonly byte[] _category = new byte[] {
            // 0 1 2 3 4 5 6 7 8 9 A B C D E F 0 1 2 3 4 5 6 7 8 9 A B C D E F 
               0,0,0,0,0,0,0,0,0,X,X,0,X,X,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
            //   ! " # $ % & ' ( ) * + , - . / 0 1 2 3 4 5 6 7 8 9 : ; < = > ? 
               X,0,0,Z,S,0,0,0,S,S,Q,Q,0,0,S,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,Q,
            // @ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _
               0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,S,S,0,S,0,
            // ' a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~ 
               0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,Q,S,0,0,0};

        internal static string EscapeExcluding(string input, params char[] exclude)
        {
            if (exclude is null)
                throw new ArgumentNullException("exclude");

            for (int i = 0; i < input.Length; i++)
            {
                if (IsMetachar(input[i]) && !exclude.Contains(input[i]))
                {
                    StringBuilder sb = new StringBuilder();
                    char ch = input[i];
                    int lastpos;

                    sb.Append(input, 0, i);
                    do
                    {
                        sb.Append('\\');
                        switch (ch)
                        {
                            case '\n':
                                ch = 'n';
                                break;
                            case '\r':
                                ch = 'r';
                                break;
                            case '\t':
                                ch = 't';
                                break;
                            case '\f':
                                ch = 'f';
                                break;
                        }
                        sb.Append(ch);
                        i++;
                        lastpos = i;

                        while (i < input.Length)
                        {
                            ch = input[i];
                            if (IsMetachar(ch) && !exclude.Contains(input[i]))
                                break;

                            i++;
                        }

                        sb.Append(input, lastpos, i - lastpos);

                    } while (i < input.Length);

                    return sb.ToString();
                }
            }

            return input;
        }

        internal static bool IsMetachar(char ch)
        {
            return (ch <= '|' && _category[ch] >= E);
        }
    }
}