What is the method for utilizing regular expressions in a subordinate directory name to retrieve files from an AWS S3 bucket in Java?

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To use regular expressions to retrieve files from an AWS S3 bucket, you can use the ListObjectsRequest class in the AWS SDK for Java. You can set the prefix parameter of this class to the name of the subordinate directory and use regular expressions to filter the results.

Here is an example:

// import AWS SDK classes
import com.amazonaws.services.s3.AmazonS3;
import com.amazonaws.services.s3.AmazonS3ClientBuilder;
import com.amazonaws.services.s3.model.ListObjectsRequest;
import com.amazonaws.services.s3.model.S3ObjectSummary;

// create AmazonS3 client
AmazonS3 s3client = AmazonS3ClientBuilder.defaultClient();

// specify the bucket name and subordinate directory name
String bucketName = "your-bucket-name";
String directoryName = "subordinate/directory";

// create a regular expression pattern to filter the results
String regexPattern = ".*\\.txt"; // retrieve only files with .txt extension

// create a ListObjectsRequest with the prefix and delimiter parameters
ListObjectsRequest listRequest = new ListObjectsRequest()
    .withBucketName(bucketName)
    .withPrefix(directoryName)
    .withDelimiter("/");

// iterate over the S3 object summaries and filter the results with the regular expression
for (S3ObjectSummary objectSummary : s3client.listObjects(listRequest).getObjectSummaries()) {
    if (objectSummary.getKey().matches(regexPattern)) {
        System.out.println(objectSummary.getKey());
        // do something with the file
    }
}

In this example, the ListObjectsRequest is created with the prefix parameter set to the subordinate directory name and the delimiter parameter set to /. This will only retrieve objects that have the specified prefix and are in the specified directory.

The regular expression pattern is specified with the regexPattern variable and is used to filter the results by matching the key (file name) of each object summary.

You can modify the regular expression pattern to match the specific file names or extensions that you want to retrieve.