Question

I have plain text file (.txt) which has some expressions and I want to replace invalid expression with… …let’s say zero.

An invalid expression is expression which contains 1+3^4^5 or 10+3+4*5+2^5^6^7, i.e. an expression can’t contain number^number^number^..., it can only contain number^number.

I understand that best way to do this is to use Regex but I dont know how to write this in regex.

Answer 1

The regular expression that will detect multiple powers is

(\d+\^){2,}

({2,} means two or more times in a row)

---

Just ran the following Test:

using System;
using System.Text.RegularExpressions;
namespace SampleNamespace
{
    public class SampleClass
    {
        public static void Main()
        {
            string line = "1+3^4^5  10+3+4*5+2^5^6^7";              
            System.Console.WriteLine(line);
            line = Regex.Replace(line, @"(\d+\^){2,}", "0");
            System.Console.WriteLine(line);
        }
    }
}

Output was:

>RegexTest.exe
1+3^4^5  10+3+4*5+2^5^6^7
1+05  10+3+4*5+07

It failed to replace the trailing \d but it does work. You can grab the trailing \d with the following correction to the regex:

(\d+\^){2,}\d+

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If you want to wipe out the entire expression that contains a double power just use

.*(\d+\^){2,}.* 

in your replace expresion. The .* on either side will swallow up the entire string surrounding the double power when a replacement takes place.