Semantic HTML

/* You are given an HTML code, written in the old non-semantic style using tags like <div id="header">, <div class="section">, etc. Your task is to write a program that converts this HTML to semantic HTML by changing tags like <div id="header"> to their semantic equivalent like <header>.
 * The non-semantic tags that should be converted are always <div>s and have either id or class with one of the following values: "main", "header", "nav", "article", "section", "aside" or "footer". Their corresponding closing tags are always followed by a comment like <!-- header -->, <!-- nav -->, etc. staying at the same line, after the tag.
 * In all converted tags you should replace multiple spaces (like <header      style="color:red">) with a single space and remove excessive spaces at the end (like <footer      >). */

namespace Semantic_HTML
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Text.RegularExpressions;

    class SemanticHTML
    {
        private const string OpenTagsPattern =
@"<(div)\s.*?((?:id|class)\s*=\s*""([a-z]+)"").*?>";

        private const string CloseTagsPattern = @"<\/div>\s+<!--\s*([a-z]+)\s*-->";
        private static HashSet<string> semanticTags;

        static void Main()
        {
            semanticTags = new HashSet<string>() { "main", "header", "nav", "article", "section", "aside", "footer" };

            string inputLine = Console.ReadLine();
            while (inputLine != "END")
            {
                if (Regex.IsMatch(inputLine, OpenTagsPattern))
                {
                    ReplaceOpenTags(inputLine);
                }
                else if (Regex.IsMatch(inputLine, CloseTagsPattern))
                {
                    ReplaceCloseTags(inputLine);
                }
                else
                {
                    Console.WriteLine(inputLine);
                }

                inputLine = Console.ReadLine();
            }
        }

        private static void ReplaceOpenTags(string htmlLine)
        {
            MatchCollection openTags = Regex.Matches(htmlLine, OpenTagsPattern);
            foreach (Match openTag in openTags)
            {
                string semanticTag = openTag.Groups[3].Value;
                if (!semanticTags.Contains(semanticTag))
                {
                    Console.WriteLine(htmlLine);
                }
                else
                {
                    string divTag = openTag.Groups[1].Value;
                    string attribute = openTag.Groups[2].Value;

                    StringBuilder sb = new StringBuilder(openTag.Value);
                    sb.Replace(divTag, semanticTag).Replace(attribute, string.Empty);
                    string result = Regex.Replace(sb.ToString(), @"\s+>", ">");
                    result = Regex.Replace(result, @"\s{2,}", " ");

                    Console.WriteLine(htmlLine.Replace(openTag.Value, result));
                }
            }
        }

        private static void ReplaceCloseTags(string htmlLine)
        {
            MatchCollection closeTags = Regex.Matches(htmlLine, CloseTagsPattern);

            htmlLine = closeTags.Cast<Match>().Where(closeTag => semanticTags.Contains(closeTag.Groups[1].Value)).Aggregate(htmlLine, (current, closeTag) => current.Replace(closeTag.Value, string.Format("</{0}>", closeTag.Groups[1].Value)));

            Console.WriteLine(htmlLine);
        }
    }
}