Custom validation with bootstrap V5

Question

So right now im basically using the default Bootstrap V5 form validator and I was wondering if there is a way to make a custom parameter that needs to be checked in order for the input to be valid. Right now I want the user to input their licenseplate, which must include letters and numbers. I more or less got the must include part, but bootstrap still says that the input is valid, even if its technically not, because it only checks if there is any input in the field. Would there be a way for me to change what bootstrap views as valid?

The default bootstrap validation function looks like this:

// Example starter JavaScript for disabling form submissions if there are invalid fields
(function () {
  'use strict'

  // Fetch all the forms we want to apply custom Bootstrap validation styles to
  var forms = document.querySelectorAll('.needs-validation')

  // Loop over them and prevent submission
  Array.prototype.slice.call(forms)
    .forEach(function (form) {
      form.addEventListener('submit', function (event) {
        if (!form.checkValidity()) {
          event.preventDefault()
          event.stopPropagation()
        }

        form.classList.add('was-validated')
      }, false)
    })
})()

Answer 1

After just spending the better part of the afternoon researching the same topic, there is very little information on custom JS validation with the OOB Bootstrap validation. I've concluded there are two viable options:

1. Using the pattern attribute on the element
2. Calling your own JS validation as part of the checkValidity() expression

Pattern Attribute

When creating an input, you can set a pattern attribute on the element.

From the MDN pattern article:

<p>
 <label>Enter your phone number in the format (123) - 456 - 7890
  (<input name="tel1" type="tel" pattern="[0-9]{3}" placeholder="###" aria-label="3-digit area code" size="2"/>) -
   <input name="tel2" type="tel" pattern="[0-9]{3}" placeholder="###" aria-label="3-digit prefix" size="2"/> -
   <input name="tel3" type="tel" pattern="[0-9]{4}" placeholder="####" aria-label="4-digit number" size="3"/>
 </label>
</p>

Here we have 3 sections for a north American phone number with an implicit label encompassing all three components of the phone number, expecting 3-digits, 3-digits and 4-digits respectively, as defined by the pattern attribute set on each.

JS Validation

When invoking the checkValidity() method, inject custom JS validation to invalidate the form's submission.

const myValidation = someValidation()

if (!myValidation || !form.checkValidity()) {
  event.preventDefault()
  event.stopPropagation()
}

When invoking someValidation(), be sure to add/remove the validation classes as-needed:

// valid input
const foo = document.getElementById("foo")
foo.classList.remove("is-invalid")
foo.classList.add("is-valid")

// invalid input
const bar = document.getElementById("bar")
bar.classList.add("is-invalid")
bar.classList.remove("is-valid")

However, I haven't quite figured out how to completely prevent the validation framework from showing the input as valid (checkmark on the right side of the input) even though the classes are set appropriately. I suspect it has something to do with the pattern attribute being empty, which simply means the field cannot be empty, and also needing to invoke the element's reportValidity() method: HTML Spec: reportValidity()

Conclusion

Looks like the pattern attribute and RegEx is the path forward when using this validation framework.

Hopefully someone else can improve this answer with a better path forward using the pure JS method.