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What is regular expressions in Java?
Can you give me an example on how to use regular expressions in Java?
Sure, here's an example: 

```
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegexExample {
   public static void main(String args[]) {
      String input = "Hello, this is a test string";
      String pattern = "test";
      Pattern p = Pattern.compile(pattern);
      Matcher m = p.matcher(input);
      if(m.find()) {
         System.out.println("Found match at index " + m.start());
      } else {
         System.out.println("No match found.");
      }
   }
}
```
In this example, we're searching for the string "test" within the input string "Hello, this is a test string". The Pattern class creates a regular expression pattern, and the Matcher class applies the pattern to the input string to find the match.