gistfile1.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;

namespace SimpleParser {

class Parser {
	public Parser(string Text) {
		Input  = Text;
	}

#region Lexer Section
	String Input = null;
	public int Position = 0;
	String Token = null;
	int    LastPosition = -1;
	int    SpaceLength = 0;
	string CurrentString;

	void SkipSpacesComments() {
		if (Position == LastPosition) return;
		LastPosition = Position;
		CurrentString = Input.Substring(Position);
		string RegEx = "^([ \\t\\n\\r]|\\/\\*.*?\\*\\/)*";
		Match M = Regex.Match(CurrentString, RegEx);
		SpaceLength = M.Length;
		if (SpaceLength > 0)
			CurrentString = CurrentString.Substring(SpaceLength);
	}
	bool R(string RegEx) {
		SkipSpacesComments();
		Match M = Regex.Match(CurrentString, "^" + RegEx, RegexOptions.IgnoreCase);
		if (M.Success) {
			Token = CurrentString.Substring(0, M.Length);
			Position += M.Length + SpaceLength;
		}
		return M.Success;
	}
	bool W(string C) {
		return R(C + "\\b"); // whole words only
	}
	bool C(string C) {
		return R(Regex.Escape(C)); 
	}

#endregion

#region Parser Section
	public bool Stopping = false;

	bool P(rule R) {
		if (Stopping) return false;
		int StartingPosition = Position;
		bool Result = R();
		if (!Result && StartingPosition != Position)
			Stopping = true;
		return Result;
	}
	
	delegate bool rule();
	delegate bool ruleL(int L);

#endregion

#region grammar section
 	ruleL expr, expr2;
	rule wexpr, mexpr, whexp, whexp2, id2, list, list2;
	rule NUMBER, ID, EOS, NIL;
	public bool Parse() {

		wexpr  = ()  => P(() => expr(0) && EOS()			  );
		
		expr   = (L) => P(() => mexpr() && expr2(L)			  );
		
		expr2  = (L) => P(() => L < 4 && C("+")   && expr(4) && expr2(L)  )
		             || P(() => L < 4 && C("-")   && expr(4) && expr2(L)  )
		             || P(() => L < 5 && C("*")   && expr(5) && expr2(L)  )
		             || P(() => L < 5 && C("/")   && expr(5) && expr2(L)  )
		             || P(() => L < 2 && C("<=")  && expr(2) && expr2(L)  )
		             || P(() => L < 2 && C(">=")  && expr(2) && expr2(L)  )
		             || P(() => L < 2 && C("<>")  && expr(2) && expr2(L)  )
		             || P(() => L < 2 && C("=")   && expr(2) && expr2(L)  )
		             || P(() => L < 2 && C("<")   && expr(2) && expr2(L)  )
		             || P(() => L < 2 && C(">")   && expr(2) && expr2(L)  )
		             || P(() => L < 1 && W("AND") && expr(1) && expr2(L)  )
		             || P(() => L < 1 && W("OR")  && expr(1) && expr2(L)  )
		             || P(() => NIL()					  );
		      
		mexpr  = ()  => P(() => C("-")    && expr(9)			  )
		             || P(() => C("+")    && expr(9)			  )
		             || P(() => W("NOT")  && expr(9)			  )
		             || P(() => C("(")    && expr(0) && C(")")		  )
		             || P(() => W("CASE") && whexp() && W("END")	  )
		             || P(() => NUMBER()				  )
		             || P(() => ID()      && id2()			  );
		
		whexp  = ()  => P(() => W("WHEN") && expr(0) && W("THEN") && expr(0) && whexp2()  );
		
		whexp2 = ()  => P(() => whexp()   				  )
		             || P(() => W("ELSE") && expr(0)			  )
		             || P(() => NIL()	  				  );
		
		id2    = ()  => P(() => C("(") && list() && C(")")  		  )
		             || P(() => NIL()					  );
		
		list   = ()  => P(() => expr(0) && list2()			  )
		             || P(() => NIL()					  );
		
		list2  = ()  => P(() => C(",") && expr(0) && list2()		  )
		             || P(() => NIL()					  );

		ID     = () => R("[a-z][a-z0-9_]*");

		NUMBER = () => R("([0-9]+(\\.[0-9]*|)|\\.[0-9]+)");

		EOS    = () => R("$");

		NIL    = () => true;

		return wexpr();
	}
#endregion
}

}