What is the method for creating a javascript string that permits the entry of only first and last names in a text field box of a contact form?

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You can create a javascript function that validates the input of the text field box, allowing only letters and spaces for the first and last name. Here is an example:

function validateName(inputText) {
  var nameFormat = /^[a-zA-Z\s]+$/;
  return nameFormat.test(inputText);
}

Then, you can use this function to check the input from the text field box and provide feedback to the user. For example:

var nameInput = document.getElementById("name");
var feedback = document.getElementById("feedback");

nameInput.addEventListener("blur", function() {
  if (!validateName(nameInput.value)) {
    feedback.innerHTML = "Please enter a valid first and last name";
    nameInput.classList.add("invalid");
  } else {
    feedback.innerHTML = "";
    nameInput.classList.remove("invalid");
  }
});

In this code, the validateName function checks if the input matches the regular expression for first and last names. If it doesn't match, the feedback element displays an error message and the invalid class is added to the nameInput element to highlight it. If it does match, the error message is cleared and the invalid class is removed.