Validate a Boolean expression with brackets in C#

Question

I want to validate a string in C# that contains a Boolean expression with brackets. The string should only contain numbers 1-9, round brackets, \"OR\" , \"AND\".

Exa

Answer 1

If you just want to validate the input string, you can write a simple parser. Each method consumes a certain kind of input (digit, brackets, operator) and returns the remaining string after matching. An exception is thrown if no match can be made.

public class ParseException : Exception { }

public static class ExprValidator
{
    public static bool Validate(string str)
    {
        try
        {
            string term = Term(str);
            string stripTrailing = Whitespace(term);

            return stripTrailing.Length == 0;
        }
        catch(ParseException) { return false; }
    }

    static string Term(string str)
    {
        if(str == string.Empty) return str;
        char current = str[0];

        if(current == '(')
        {
            string term = LBracket(str);
            string rBracket = Term(term);
            string temp = Whitespace(rBracket);
            return RBracket(temp);
        }
        else if(Char.IsDigit(current))
        {
            string rest = Digit(str);
            try
            {
                //possibly match op term
                string op = Op(rest);
                return Term(op);
            }
            catch(ParseException) { return rest; }
        }
        else if(Char.IsWhiteSpace(current))
        {
            string temp = Whitespace(str);
            return Term(temp);
        }
        else throw new ParseException();
    }

    static string Op(string str)
    {
        string t1 = Whitespace_(str);
        string op = MatchOp(t1);
        return Whitespace_(op);
    }

    static string MatchOp(string str)
    {
        if(str.StartsWith("AND")) return str.Substring(3);
        else if(str.StartsWith("OR")) return str.Substring(2);
        else throw new ParseException();
    }

    static string LBracket(string str)
    {
        return MatchChar('(')(str);
    }

    static string RBracket(string str)
    {
        return MatchChar(')')(str);
    }

    static string Digit(string str)
    {
        return MatchChar(Char.IsDigit)(str);
    }

    static string Whitespace(string str)
    {
        if(str == string.Empty) return str;

        int i = 0;
        while(i < str.Length && Char.IsWhiteSpace(str[i])) { i++; }

        return str.Substring(i);
    }

    //match at least one whitespace character
    static string Whitespace_(string str)
    {
        string stripFirst = MatchChar(Char.IsWhiteSpace)(str);
        return Whitespace(stripFirst);
    }

    static Func MatchChar(char c)
    {
        return MatchChar(chr => chr == c);
    }

    static Func MatchChar(Func pred)
    {
        return input => {
            if(input == string.Empty) throw new ParseException();
            else if(pred(input[0])) return input.Substring(1);
            else throw new ParseException();
        };
    }
}