PostcodeText.php

<?php
namespace Parts\Content;

use Parts\Primitive\StringType;
use Parts\Restrictions\MaxLength;
use Parts\Restrictions\MinLength;
use Parts\Restrictions\Pattern;

/**
 * Class PostcodeText
 * Holds a UK postcode.
 * @package Parts\Content
 */
class PostcodeText extends StringType implements MaxLength, MinLength, Pattern
{
    public function __construct($string)
    {
        $string = $this->convertType($string);
        
        $this->validateMinLength($string, 1);
        $this->validateMaxLength($string, 8);
        $this->validateRegEx($string, '#^[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][A-Z]{2}$#');
        
        parent::__construct($string);
    }
}

StringType.php

<?php
namespace Parts\Primitive;

use Parts\InvalidPrimitiveTypeException;
use Parts\Restrictions\Enumeration;
use Parts\Restrictions\MaxLength;
use Parts\Restrictions\MinLength;
use Parts\Restrictions\Pattern;
use Parts\ValidationRestrictionException;

/**
 * The base type to represent a string.
 * @package Parts\Primitive
 */
class StringType extends BaseSimpleType
{
    /**
     * Convert the data into a more suitable form.
     * @param mixed $var
     * @return string
     */
    protected final function convertType($var): string
    {
        /**
         * Possible php string conversions.
         * Props to user mpen of StackOverflow
         * @link http://stackoverflow.com/a/27368848/2274710
         */
        if ($var === null || is_scalar($var) || is_callable([$var, '__toString']))         {
            return (string)$var;
        }

        throw new InvalidPrimitiveTypeException(sprintf(
            '%s expected a string or type which can be converted to string, %s given.',
            get_class($this),
            gettype($var)
        ));
    }

    protected final function validateMaxLength(string $string, int $maxLength)
    {
        $length = strlen($string);

        if ($length <= $maxLength) {
            return;
        }

        throw new ValidationRestrictionException(sprintf(
            '%s expected a string of at most %s character%s, %s given.',
            get_class($this),
            $maxLength,
            $maxLength === 1 ? '' : 's',
            $length
        ));
    }
    
    protected final function validateMinLength(string $string, int $minLength)
    {
        $length = strlen($string);

        if ($length >= $minLength) {
            return;
        }

        throw new ValidationRestrictionException(sprintf(
            '%s expected a string of at least %s character%s, %s given.',
            get_class($this),
            $maxLength,
            $maxLength === 1 ? '' : 's',
            $length
        ));
    }
    
    protected final function validateRegEx(string $string, string $pattern)
    {
        if (preg_match($pattern, $string)) {
            return;
        }

        throw new ValidationRestrictionException(sprintf(
            '%s expected a string matching the pattern ' . "\n" . '%s.',
            get_class($this),
            $pattern
        ));
    }
    
    protected final function validateEnum(string $string, string $pattern)
    {
        $choices = $this->getEnumerableChoices();

        if (in_array($string, $choices, true)) {
            return;
        }

        throw new ValidationRestrictionException(sprintf(
            '%s expected one of the following choices: %s; "%s" given',
            get_class($this),
            implode(', ', $choices),
            $string
        ));
    }

    /**
     * StringType constructor.
     * @param string $string A text string.
     */
    public function __construct($string)
    {
        parent::__construct($this->convertType($string));
    }
}