Q:

Javascript date validator for multiple date patterns

With help from stackoverflow I made a .js file with two methods that check if the date entered is valid.  It is working fine, but only if the date is entered with dashes - 12/12/2019.  What approach can I take in my isValidDate method to make it work with multiple date patterns like - 12.12.2020  12-12-2020 etc.  The only idea I have is to check somehow first what is the pattern and then create a separate case to split the date string by different characters, but it will look pretty ugly. 
function isValidDate(dateString) {
var validDate = true;

// First check for the pattern
if (! /^\d{1,2}\/\d{1,2}\/\d{4}$/.test(dateString)) {
    return false;
}

// Parse the date parts to integers
var parts = dateString.split("/");
var day = parseInt(parts[0], 10);
var month = parseInt(parts[1], 10);
var year = parseInt(parts[2], 10);

if(isNaN(day)  || isNaN(month) || isNaN(year)){
    return false;
}

// Check the ranges of month and year
if (year < 1000 || year > 3000 || month < 1 || month > 12) {
    return false;
}

var monthLength = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

// Adjust for leap years
if (year % 400 == 0 || (year % 100 ! = 0 && year % 4 == 0)) {
    monthLength[1] = 29;
}

// Check the range of the day
if (!( day > 0 && day <= monthLength[month - 1])) {
    return false;
}

    return true;

};

 /**
 * Function which executes each time the input loses focus (onblur)
 */
    function validateDate() {
    // Get a reference to the container
    var container = document.querySelector('.date');

    // Clear stale error/success messages
    container.className = container.className.replace('success', '');
    container.className = container.className.replace('error', '');

    // Get current value of input
    var dateString = document.getElementById("Start-date").value;

    // Test if the string is a valid date
    var isValid = isValidDate(dateString);

    // Update classess of container to show success/error
    if (! isValid) {
    container.className += ' error';
    } else {
    container.className += ' success';
    }
    } 

I am calling the validateDate() function first 

A:

Please note I have not tested the regular expressions this is just an example of how to attack the probelm

// First check for the pattern var = dateChar
if (! /^\d{1,2}\/\d{1,2}\/\d{4}$/.test(dateString)) {
    dateChar = '/'; 
}

if (! /^\d{1,2}\-\d{1,2}\-\d{4}$/.test(dateString)) {
    dateChar = '-'; 
}

if (! dateChar) {
  return false; 
}

var parts = dateString.split(dateChar);

....

