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<html><body><h1>The input pattern attribute</h1><p>The pattern attribute specifies a regular expression that the input element's value is checked against.</p><form 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" value="Submit"></form></body></html>