I don't like to use keywords like category and article in the URLs. That's the reason I am using the following route for my blog with embedded child routes. It makes easier using the same controller with different actions for blog index, category index and article.

<?php
namespace Application;

use Zend\Router\Http\Literal;
use Zend\Router\Http\Segment;

return [
    // ...
    
    'router' => [
        'routes' => [
            'blog' => [
                'type' => Literal::class,
                'options' => [
                    'route'    => '/blog/',
                    'defaults' => [
                        'controller' => Controller\BlogController::class,
                        'action'     => 'index',
                    ],
                ],
                'may_terminate' => true,
                'child_routes' => [
                    'category' => [
                        'type' => Segment::class,
                        'options' => [
                            'route' => ':category[/:page]/',
                            'constraints' => [
                                'category' => '[a-zA-Z][a-zA-Z0-9_-]*',
                                'page' => '[0-9]+',
                            ],
                            'defaults' => [
                                'action' => 'category',
                            ],
                        ],
                        'may_terminate' => true,
                        'child_routes' => [
                            'article' => [
                                'type' => Segment::class,
                                'options' => [
                                    'route' => ':article/',
                                    'constraints' => [
                                        'article' => '[a-zA-Z][a-zA-Z0-9_-]*',
                                    ],
                                    'defaults' => [
                                        'action' => 'article',
                                    ],
                                ],
                            ],
                        ]
                    ],
                ]
            ],
        ],
    ],
	
	// ...
],

However, the child route with the parent child route having [:page] parameter caused following error even it is an optional parameter where I have URL helper trying to create a url for any article link.

To solve this problem, I used a second first level child route instead inner one for the article. I considered the fact that I don't have an article alias consist of integer only. This way the blog/category route is only responsible for the paging since it is tracked by the [:page] numeric value while the blog/article route monitors article urls called by category and article aliases.

Here is the revised routing:

<?php
namespace Application;

use Zend\Router\Http\Literal;
use Zend\Router\Http\Segment;

return [
    // ...
    
    'router' => [
        'routes' => [
            'blog' => [
                'type' => Literal::class,
                'options' => [
                    'route'    => '/blog/',
                    'defaults' => [
                        'controller' => Controller\BlogController::class,
                        'action'     => 'index',
                    ],
                ],
                'may_terminate' => true,
                'child_routes' => [
                    'category' => [
                        'type' => Segment::class,
                        'options' => [
                            'route' => ':category[/:page]/',
                            'constraints' => [
                                'category' => '[a-zA-Z][a-zA-Z0-9_-]*',
                                'page' => '[0-9]+',
                            ],
                            'defaults' => [
                                'action' => 'category',
                                'page' => 1,
                            ],
                        ],
                    ],
                    'article' => [
                        'type' => Segment::class,
                        'options' => [
                            'route' => '/:category/:article/',
                            'constraints' => [
                                'category' => '[a-zA-Z][a-zA-Z0-9_-]*',
                                'article' => '[a-zA-Z][a-zA-Z0-9_-]*',
                            ],
                            'defaults' => [
                                'action' => 'article',
                            ],
                        ],
                    ],
                ],
            ],
        ],
    ],
	
    // ...	
],