Convert certificate string to byte array

IMPORTANT

As @dave_thompson_085 has pointed out in the comments, SunJCE CertificateFactory is indeed capable of parsing PEM files.

So you can just use that to get the Certificate object as detailed at How to load public certificate from pem file..? (which is an earlier answer by @dave on the same topic, so please upvote it, instead of this one, if you find this useful !), and then access its encoded (DER) form.

However, if your PEM file is a raw "RSA PUBLIC KEY" (like the one that was attached to this question), or some other entity which SunJCE implementation can not parse directly, you can still parse and decode it manually, as detailed below.

Technically what you have here is not a certificate, but just a public key.

You can decode it to DER bytes as simple as that:

byte[] derBytes = Base64.getDecoder().decode(
    pemText.replaceAll("-----(BEGIN|END) RSA PUBLIC KEY-----", "").replaceAll("\n", "")
);

Note, that what you will get will be a raw RSA (PKCS#1) key:

RSAPublicKey ::= SEQUENCE {
    modulus           INTEGER,  -- n
    publicExponent    INTEGER   -- e
}

You can use the same technique to decode X.509 certificates or private keys.

E.g. the code to decode the X.509 certificate:

byte[] certificateBytes = Base64.getDecoder().decode(
    pemText.replaceAll("-----(BEGIN|END) CERTIFICATE-----", "").replaceAll("\n", "").getBytes("UTF-8")
);

CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
    X509Certificate certificate = (X509Certificate)(certificateFactory.generateCertificate(
        new ByteArrayInputStream(certificateBytes)
    )
);

UPDATE

The code above uses Java 8 Base64 decoder. As question has been updated asking for a Java 7 solution, here is a link to an excellent thread, discussing various options available: Base64 Java encode and decode a string.

E.g. java.xml.bind method described there does not require any extra libraries on Java 7 (which seems to match what OP wants)