revision:
The pattern attribute specifies a regular expression that the <input> element's value is checked against. The pattern attribute can be used on the following element: <input>. The pattern attribute works with the following input types: text, date, search, url, tel, email, and password.
Tip: use the global title attribute to describe the pattern to help the user.
<input pattern="regular expression">
A regular expression is a formalized string of characters that define a pattern. For example [a-zA-Z0-9]+ is a pattern that matches against a string of any length, as long as the string contains only lowercase letters (a-z), uppercase letters (A-Z), or numerals (0-9).
<form style="margin-left:3vw;" action="/action_page.php">
<label for="country_code">Country code:</label>
<input type="text" id="country_code" name="country_code"
pattern="[A-Za-z]{3}" title="Three letter country code"><br><br>
<input type="submit">
</form>
A form with a password field that must contain 8 or more characters that are of at least one number, and one uppercase and lowercase letter:
<p class="spec">A form with a password field that must contain 8
or more characters that are of at least one number,
and one uppercase and lowercase letter:</p>
<form form style="margin-left:3vw;" action="/action_page.php">
<label for="pwd">Password:</label>
<input type="password" id="pwd" name="pwd"
pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,}" title="Must contain at least one number
and one uppercase and lowercase letter, and at least 8 or more characters">
<input type="submit">
</form>