RegEx Highlighter

Highlighter

public class Highlighter
{
	private const string TempStartToken = "\x02";
	private const string TempEndToken = "\x03";

	private const string SearchPatternTemplate = $"[{TempStartToken}{TempEndToken}]*{{0}}";
	private const string ReplacePattern = $"{TempStartToken}$&{TempEndToken}";

	private readonly ConcurrentDictionary<HighlightKey, Regex> _regexPatternsCache = new();

	private static string GetHighlightTypeTemplate(HighlightType highlightType) =>
		highlightType switch
		{
			HighlightType.Starts => "^{0}",
			HighlightType.Contains => "{0}",
			HighlightType.Ends => "{0}$",
			HighlightType.Equals => "^{0}$",
			_ => throw new ArgumentException($"Unsupported {nameof(HighlightType)}: '{highlightType}'", nameof(highlightType)),
		};

	public string Highlight(string text, IReadOnlySet<string> words, string startToken, string endToken, HighlightType highlightType)
	{
		foreach (var word in words)
		{
			var key = new HighlightKey
			{
				Word = word,
				HighlightType = highlightType,
			};

			var regex = _regexPatternsCache.GetOrAdd(key, _ =>
			{
				var parts = word.Select(w => string.Format(SearchPatternTemplate, Regex.Escape(w.ToString())));
				var pattern = string.Concat(parts);
				var highlightPattern = string.Format(GetHighlightTypeTemplate(highlightType), pattern);

				return new Regex(highlightPattern, RegexOptions.IgnoreCase | RegexOptions.CultureInvariant | RegexOptions.Compiled);

			});
			
			text = regex.Replace(text, ReplacePattern);
		}

		return text
			.Replace(TempStartToken, startToken)
			.Replace(TempEndToken, endToken)
		;
	}

	private record HighlightKey
	{
		public string Word { get; init; }
		public HighlightType HighlightType { get; init; }
	}
}

public enum HighlightType
{
	Starts,
	Contains,
	Ends,
	Equals,
}

void Main()
{
	var queries = new[] { "abc" }.ToHashSet();
	var search = "a ab abc abcd abcde";

	var highlighter = new Highlighter();
	var outputs = search
		.Split((string[])null, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
		.Select(w => highlighter.Highlight(w, queries, "<b>", "</b>", HighlightType.Starts))
	;
	
	var result = string.Join(" ", outputs).Dump();
	Util.RawHtml(result).Dump();
}