<?php declare(strict_types = 1);

$data = [
	'c1' => 12,
	'rasd' => 13,
	'c34' => 15,
];

foreach ($data as $key => $value) {
    $match = [];
    if (false === preg_match('/^c(\d+)$/', $key, $match) || empty($match)) {
        continue;
    }            
    var_dump($key);
    var_dump($value);
}