Sign string with PEM PrivateKey

I have a PEM-encoded private key and I need to sign a string with it. But the code keeps crashing with exception:

java.security.spec.InvalidKeySpecException: java.lang.RuntimeException: error:0c0890ba:ASN.1 encoding routines:asn1_check_tlen:WRONG_TAG

the key string:

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIICxjBABgkqhkiG9w0BBQ0wMzAbBgkqhkiG9w0BBQwwDgQI4P/+9mJV6RwCAggA 
MBQGCCqGSIb3DQMHBAg/ZWGXeLHgeASCAoAhExhFxfcikmIKbFP0rgAlJuj1r999 
... and so on...
hlgzM2p71FdC6NDVyyxbit/IzbimtJyhkRwOAnZ98yqtXWUEOx2v7CcUqiU8dSLA 
K0PsaxNTUeUcQV+Z7yJk/8HxfE1ya3u2CgPXCZsWWmbxQG/+awE0eEnZ 
-----END ENCRYPTED PRIVATE KEY-----

I've tried many variants, looked through many answers, but the results were the same

Edit: with James K Polk's help I've managed to get private key bytes, but now I get java.security.NoSuchAlgorithmException: SecretKeyFactory PBES2 implementation not found. Modified code:

private String sign(String dataString, String pkString, String privateKeyPass) throws Exception {
        pkString = pkString.replace("-----BEGIN ENCRYPTED PRIVATE KEY-----", "");
        pkString = pkString.replace("-----END ENCRYPTED PRIVATE KEY-----", "");
        pkString = pkString.replaceAll("\\s+","");
        byte[] privateKeyBytes = decryptPrivateKey(Base64.decode(pkString, Base64.DEFAULT), privateKeyPass);
        KeyFactory kf = KeyFactory.getInstance("RSA");
        PrivateKey privateKey = kf.generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));

        Signature instance = Signature.getInstance("SHA1withRSA");
        instance.initSign(privateKey);
        instance.update(dataString.getBytes(UTF_8));
        return Base64.encodeToString(instance.sign(), Base64.DEFAULT);
    }

public static byte[] decryptPrivateKey(byte[] key, String pass) throws Exception {
        PBEKeySpec passKeySpec = new PBEKeySpec(pass.toCharArray());

        EncryptedPrivateKeyInfo encryptedKey = new EncryptedPrivateKeyInfo(key);
        Timber.w("encryptedKey.getAlgName(): %s", encryptedKey.getAlgName());
        SecretKeyFactory keyFac = SecretKeyFactory.getInstance(encryptedKey.getAlgName());//PBES2
        SecretKey passKey = keyFac.generateSecret(passKeySpec);

        // Create PBE Cipher
        Cipher pbeCipher = Cipher.getInstance(encryptedKey.getAlgName());
        // Initialize PBE Cipher with key and parameters
        pbeCipher.init(Cipher.DECRYPT_MODE, passKey, encryptedKey.getAlgParameters());

        // Decrypt the private key
        return pbeCipher.doFinal(encryptedKey.getEncryptedData());
    }

EDIT: I ended up using class from http://commandlinefanatic.com/cgi-bin/showarticle.cgi?article=art050:

PrivateKey privateKey = KeyImport.readPrivateKeyFile(pkFileName, privateKeyPass);

I save keyString into a file and then feed it to readPrivateKeyFile

Your private key is encrypted according PKCS#8, so you need to use the EncryptedPrivateKeyInfo class among others. This stackoverflow question contains an example showing how to retrieve it. I have turned it into a more complete example below:

import javax.crypto.Cipher;
import javax.crypto.EncryptedPrivateKeyInfo;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.PBEKeySpec;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Base64;
import java.util.List;

public class Main {

    private static byte [] pemFileToBytes(String filename) throws IOException {
        // read in PEM file, throw away the begin and end lines
        List<String> pemLines = Files.readAllLines(Paths.get(filename), StandardCharsets.US_ASCII);
        pemLines.remove(0);
        pemLines.remove(pemLines.size() - 1);
        String pem = String.join("", pemLines);

        // base64 decode and return the result.

        return Base64.getDecoder().decode(pem);
    }

    private static PrivateKey parsePrivateKey (String filename, char [] password) throws Exception{
        PBEKeySpec passKeySpec = new PBEKeySpec(password); //my password

        EncryptedPrivateKeyInfo encryptedKey = new EncryptedPrivateKeyInfo(pemFileToBytes(filename));
        SecretKeyFactory keyFac = SecretKeyFactory.getInstance(encryptedKey.getAlgName());
        SecretKey passKey = keyFac.generateSecret(passKeySpec);

        // Create PBE Cipher
        Cipher pbeCipher = Cipher.getInstance(encryptedKey.getAlgName());
        // Initialize PBE Cipher with key and parameters
        pbeCipher.init(Cipher.DECRYPT_MODE, passKey, encryptedKey.getAlgParameters());

        // Decrypt the private key

        byte [] encodedPrivateKey = pbeCipher.doFinal(encryptedKey.getEncryptedData());
        PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(encodedPrivateKey);
        KeyFactory kf = KeyFactory.getInstance("RSA");
        return kf.generatePrivate(privateKeySpec);

    }

    public static void main(String[] args) throws Exception {
        PrivateKey pk = parsePrivateKey("x.pk8", "pass".toCharArray());
    }
}

You last line, return new String(instance.sign(), UTF_8); doesn't make any sense as Signature.sign() returns an array of bytes that is not likely to be a valid string in any character set. If you must convert the signature to a string then the standard way is to base64 encode it.