how to use regex to verify a certain string pattern?

[HFT_developer]

Hi,
My aim to check whether a user has entered the input correctly. The input is a std::string which should be a polynomial. The following are two examples of what should be valid or invalid:

    std::string sample1 = {"4x^3 - 9x^2 + 10x^1 -5x^0 -45x^-1"}; // valid
    std::string sample2 = {"4x^3 - 9x^2 + 10x -5 -45x^-1"};// invalid

Right now I am trying to use std::regex to verify this:

bool verifyPolynomial(const std::string& poly){
     // Used when your searching a string
    std::smatch matches;
    
    std::regex _regex("[0-9]+[a-z]+\\^[0-9]");// I don't think this is correct?
    
    std::regex_search(poly, matches, _regex);
    
    return matches.size();
}

I am learning regex, so I find this a bit confusing if someone can help that would be great

[JonB]

@HFT_developer
This is not an easy one for a beginner! Try playing with https://regex101.com, where you can test regular expressions against whatever input you give it.

The final answer depends on how strict you want to be, what you do/do not want to allow, whether it must match the whole, complete string, how you want to handle whitespace, and so on.

I see your attempt as close:

[0-9]+[a-z]+\^-?[0-9]+

might do you for each polynomial in the expression.

Then you may have to go:

[0-9]+[a-z]+\^-?[0-9]+(\s*[+-]\s*[0-9]+[a-z]+\^-?[0-9]+)*

to do the series of polys separated by a + or - symbol. Or something like that :)

[SGaist]

Hi,

Qt also have QRegularExpression for that kind of work and there's a helper tool that you can build to test your expressions.

[Kent-Dorfman]

mathematically speaking both expressions are valid. -5x^0 evalutates to -5 so the term is the same even if it is expressed differently.