A very simple parser combinator

dumb-parser-combinator.cs

void Main()
{
	// The text to test it on
	var text = new WorkingText("Annie1aa2");

	// The parser: hits on "Deane" or "Annie" following by a number, then two letters, then another number
	var parser = new Sequence(
		new Any(
			new TextParser("Deane"),
			new TextParser("Annie")
		),
		new AnyNumberParser(),
		new AnyLetterParser(),
		new AnyLetterParser(),
		new AnyNumberParser()
	);

	var results = new StringBuilder();
	parser.Parse(ref text, ref results);
	results.Dump();

	// results: Annie1aa2
	// textPosition: 9
}

public interface IParser
{
	bool Parse(ref WorkingText text, ref StringBuilder sb);
}

// Any string of characters
public class TextParser : IParser
{
	private string value;

	public TextParser(string _value)
	{
		value = _value;
	}

	public bool Parse(ref WorkingText text, ref StringBuilder sb)
	{
		var start = text.Position;
		var slice = text.GetNext(value.Length);

		if (slice == value)
		{
			sb.Append(slice);
			return true;
		}

		text.Position = start;
		return false;
	}
}

// Any non-numeric character
public class AnyLetterParser : IParser
{
	public bool Parse(ref WorkingText text, ref StringBuilder sb)
	{
		var character = text.GetNext();
		if (Char.IsLetter(character))
		{
			sb.Append(character);
			return true;
		}

		return false;
	}
}

// Any numeric character
public class AnyNumberParser : IParser
{
	public bool Parse(ref WorkingText text, ref StringBuilder sb)
	{
		var character = text.GetNext();
		if (Char.IsDigit(character))
		{
			sb.Append(character);
			return true;
		}

		return false;
	}
}


public class Any : IParser
{
	private IParser[] parsers;

	public Any(params IParser[] _parsers)
	{
		parsers = _parsers;
	}

	public bool Parse(ref WorkingText text, ref StringBuilder sb)
	{
		foreach (var parser in parsers)
		{
			if (parser.Parse(ref text, ref sb))
			{
				return true;
			}
		}

		// We don't need to reset here, because the failing parsers should have reset for us
		return false;
	}

}

public class Sequence : IParser
{
	private IParser[] parsers;

	public Sequence(params IParser[] _parsers)
	{
		parsers = _parsers;
	}

	public bool Parse(ref WorkingText text, ref StringBuilder sb)
	{
		var start = text.Position;
		var sbStart = sb.Length;

		foreach (var parser in parsers)
		{
			if (!parser.Parse(ref text, ref sb))
			{
				// Something returned false
				text.Position = start; // Reset the cursor position
				sb.Remove(sbStart, sb.Length);  // Result the results
				return false;
			}
		}

		return true;
	}
}

// This is basically just a string with a position indicator
public class WorkingText
{
	public string Value { get; set; }
	public int Position { get; set; }

	public WorkingText(string text)
	{
		Value = text;
	}

	public string GetNext(int length)
	{
		var result = Value.Substring(Position, length);
		Position += length;
		return result;
	}

	public char GetNext()
	{
		var character = Value[Position];
		Position++;
		return character;
	}

	public bool AtEnd()
	{
		return Position == Value.Length;
	}
}