Validation

<input> elements of type text have no automatic validation applied to them (since a basic text input needs to be capable of accepting any arbitrary string), but there are some client-side validation options available, which we'll discuss below.

Note: HTML form validation is not a substitute for server-scripts that ensure the entered data is in the proper format. It's far too easy for someone to make adjustments to the HTML that allow them to bypass the validation, or to remove it entirely. It's also possible for someone to bypass your HTML entirely and submit the data directly to your server. If your server-side code fails to validate the data it receives, disaster could strike when improperly-formatted data (or data which is too large, is of the wrong type, and so forth) is entered into your database.


A note on styling

There are useful pseudo-classes available for styling form elements to help the user see when their values are valid or invalid. These are :valid and :invalid. In this section, we'll use the following CSS, which will place a check (tick) mark next to inputs containing valid values, and a cross (X) next to inputs containing invalid values.

div {
  margin-bottom: 10px;
  position: relative;
}

input + span {
  padding-right: 30px;
}

input:invalid + span::after {
  position: absolute;
  content: "✖";
  padding-left: 5px;
}

input:valid + span::after {
  position: absolute;
  content: "✓";
  padding-left: 5px;
}

The technique also requires a <span> element to be placed after the form element, which acts as a holder for the icons. This was necessary because some input types on some browsers don't display icons placed directly after them very well.


Making input required

You can use the required attribute as an easy way of making entering a value required before form submission is allowed:

<form>
  <div>
    <label for="uname">Choose a username: </label>
    <input type="text" id="uname" name="name" required />
    <span class="validity"></span>
  </div>
  <div>
    <button>Submit</button>
  </div>
</form>

This renders like so:

If you try to submit the form with no search term entered into it, the browser will show an error message.


Input value length

You can specify a minimum length (in characters) for the entered value using the minlength attribute; similarly, use maxlength to set the maximum length of the entered value, in characters.

The example below requires that the entered value be 4–8 characters in length.

<form>
  <div>
    <label for="uname">Choose a username: </label>
    <input
      type="text"
      id="uname"
      name="name"
      required
      size="10"
      placeholder="Username"
      minlength="4"
      maxlength="8" />
    <span class="validity"></span>
  </div>
  <div>
    <button>Submit</button>
  </div>
</form>

This renders like so:

If you try to submit the form with less than 4 characters, you'll be given an appropriate error message (which differs between browsers). If you try to enter more than 8 characters, the browser won't let you.

Note: If you specify a minlength but do not specify required, the input is considered valid, since the user is not required to specify a value.


Specifying a pattern

You can use the pattern attribute to specify a regular expression that the inputted value must match in order to be considered valid (see Validating against a regular expression for a simple crash course on using regular expressions to validate inputs).

The example below restricts the value to 4-8 characters and requires that it contain only lower-case letters.

<form>
  <div>
    <label for="uname">Choose a username: </label>
    <input
      type="text"
      id="uname"
      name="name"
      required
      size="45"
      pattern="[a-z]{4,8}" />
    <span class="validity"></span>
    <p>Usernames must be lowercase and 4-8 characters in length.</p>
  </div>
  <div>
    <button>Submit</button>
  </div>
</form>

This renders like so: