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javascripthtmlvalidationinputerror-messaging

Detect the error message in the input field

How can I show the message if the user types a restricted symbol?

For example, if the user types * in the input field, the error message can show A filename cannot contain any of the following characters: \/:*?"<>|. I hope someone can guide me how to do it. Thanks.

<!DOCTYPE html>
<html>
<body>

<h1>How to show error message</h1>


<input type="text" class="form-control blank" id="function_code" name="function_code" title="function_code" onpaste="return false">

</body>
</html>

<script>
document.getElementById("function_code").onkeypress = function(e) {
var chr = String.fromCharCode(e.which);
if ("></\":*?|".indexOf(chr) >= 0)
return false;
};
</script>

My expected result is like below the picture if the user types the restrict symbol in the input field:

output1

Solution

  • Use the input event along with a regular expression, like so:

    const input = document.getElementById("function_code");
const error = document.getElementById('error');
const regex = /[\\\/:*?"<>|]+/;

input.addEventListener('input', (e) => {
  const value = e.target.value;

  if (regex.test(value)) {
    input.value = value.slice(0, value.length - 1);
    error.textContent = 'A filename cannot contain any of the following characters: \/:*?"<>|';
  } else {
    error.textContent = '';
  }
});
    input {
  padding: 8px 10px;
  border-radius: 5px;
  font-size: 1.2rem;
}

#error {
  display: block;
  color: red;
  margin: 5px 0 0 0;
}
    <input type="text" id="function_code" name="function_code">
<span id="error"></span>