Performs different checks for attribute values.

The currently implemented checks are “maxlen”, “minlen”, “maxval”, “minval”,
and “valueless”.

function wp_kses_check_attr_val( $value, $vless, $checkname, $checkvalue ) {
	$ok = true;

	switch ( strtolower( $checkname ) ) {
		case 'maxlen':
			/*
			 * The maxlen check makes sure that the attribute value has a length not
			 * greater than the given value. This can be used to avoid Buffer Overflows
			 * in WWW clients and various Internet servers.
			 */

			if ( strlen( $value ) > $checkvalue ) {
				$ok = false;
			}
			break;

		case 'minlen':
			/*
			 * The minlen check makes sure that the attribute value has a length not
			 * smaller than the given value.
			 */

			if ( strlen( $value )  $checkvalue ) {
				$ok = false;
			}
			break;

		case 'minval':
			/*
			 * The minval check makes sure that the attribute value is a positive integer,
			 * and that it is not smaller than the given value.
			 */

			if ( ! preg_match( '/^s{0,6}[0-9]{1,6}s{0,6}$/', $value ) ) {
				$ok = false;
			}
			if ( $value `) or not (``). If the given value
			 * is a "y" or a "Y", the attribute must not have a value.
			 * If the given value is an "n" or an "N", the attribute must have a value.
			 */

			if ( strtolower( $checkvalue ) != $vless ) {
				$ok = false;
			}
			break;

		case 'values':
			/*
			 * The values check is used when you want to make sure that the attribute
			 * has one of the given values.
			 */

			if ( false === array_search( strtolower( $value ), $checkvalue, true ) ) {
				$ok = false;
			}
			break;

		case 'value_callback':
			/*
			 * The value_callback check is used when you want to make sure that the attribute
			 * value is accepted by the callback function.
			 */

			if ( ! call_user_func( $checkvalue, $value ) ) {
				$ok = false;
			}
			break;
	} // End switch.

	return $ok;
}