JSFiddle - React, Tailwind, and code Playground

JavaScript

function validateNumber(thenumber)
{
    thenumber = thenumber.replace(".", "").replace(",", ".");
    
    return !isNaN(thenumber);
}

console.log(validateNumber("123"));
console.log(validateNumber("-123"));
console.log(validateNumber("123.12"));
console.log(validateNumber("-123.12"));
console.log(validateNumber("12.345,67"));
console.log(validateNumber("-12.345,67"));
console.log(validateNumber("123a"));