Olá.

Montei esse código abaixo só que não estou conseguindo retornar o hash base64 na string.

Se puder me ajudar informando onde estou errando nessa regex.

$url = 'http://teste.net/teste?cliente=TG9yZW0gSXBzdW0gw6kgc2ltcGxlc21lbnRlIHVtYSBzaW11bGHDp8OjbyBkZSB0ZXh0byBkYSBpbmTDunN0cmlhIHRpcG9ncsOhZmljYSBlIGRlIGltcHJlc3NvcywgZSB2ZW0gc2VuZG8gdXRpbGl6YWRvIGRlc2RlIG8gc8OpY3VsbyBYVkksIHF1YW5kbyB1bSBpbXByZXNzb3IgZGVzY29uaGVjaWRvIHBlZ291IHVtYSBiYW5kZWphIGRlIHRpcG9zIGUgb3MgZW1iYXJhbGhvdSBwYXJhIGZhemVyIHVtIGxpdnJvIGRlIG1vZGVsb3MgZGUgdGlwb3Mu';

preg_match('/^([A-Za-z0-9+\/]{4})*([A-Za-z0-9+\/]{4}|[A-Za-z0-9+\/]{3}=|[A-Za-z0-9+\/]{2}==)$/i', $url, $matches);

print_r($matches);