Gravity Forms - Allow certain HTML tags, remove link (a, href) tags, and prevent http and https from being submitted during validation.

functions.php

<?php

/**
 * Gravity Forms: Allowable tags.
 * By returning specific HTML tags, we can prevent link tags from being submitted.
 * The links will simply be stripped out without a warning to the user.
 *
 * @author Anything Graphic
 * @link https://anythinggraphic.net
 * @link https://community.gravityforms.com/t/how-to-block-links-in-text-field-resolved/5865/5
 * @link https://docs.gravityforms.com/gform_allowable_tags/
 * @param array $allowable_tags the string of default allowable tags, removing the anchor tag.
 */
function ag_allow_basic_tags( $allowable_tags ) {
	return '<p><strong><em>'; // No link tag, only excepting paragraphc, strong, and emphasis HTML tags.
}
add_filter( 'gform_allowable_tags', 'ag_allow_basic_tags' );




/**
 * Gravity Forms: Form Validation.
 *
 * The below looks for the words 'http' and 'https' upon after the user tries to submit the form.
 * During form validation, if the words appear within the form, the form will not submit.
 * The user will then be presented with a custom, informative, and actionable message.
 *
 * @author Anything Graphic
 * @link https://anythinggraphic.net
 * @link https://docs.gravityforms.com/gform_field_validation
 * @param array  $result The validation result to be filtered.
 * @param string $value The field value to be validated. Multi-input fields like Address will pass an array of values.
 * @param object $form Current form object.
 * @param object $field Current field object.
 */
function ag_disable_urls_validation( $result, $value, $form, $field ) {
	$nourl_pattern = '(http|https)'; // Looks for http and https.
	if ( preg_match( $nourl_pattern, $value ) ) {
		$result['is_valid'] = false;
		$result['message']  = 'Message can not contain website addresses.';
	}

	return $result;
}
add_filter( 'gform_field_validation', 'ag_disable_urls_validation', 10, 4 );