# Apache Beam: Regex

## Overview

Do you have a custom DoFn that applies a regular expression (regex) pattern to the input element? Well, did you know there is a built-in Apache Beam transform called `Regex` which can simplify your code?

## When You Should Use the Keys Transform

You should use the `Regex` transform if you want to perform the following for every element in a PCollection:

1. Filter for strings matching a certain pattern
    
2. Replace a pattern of a string with another string
    
3. Split a string with a specific delimiter
    

## How to Use the Keys Transform

Just apply the built-in transform to a PCollection of strings. Use `Regex.Matches` to filter input elements based on a certain regex pattern. Use `Regex.ReplaceAll` to replace substrings in a string. Use `Regex.Split` to split a string into multiple strings using a delimiter.

## Example: Filter Strings with Email Regex Pattern

```java
// Create a collection with strings
    PCollection<String> emails =
        pipeline.apply(
            Create.of(
                "johndoe@gmail.com",
                "sarahsmith@yahoo.com",
                "mikebrown@outlook.com",
                "amandajohnson",
                "davidlee",
                "emilyrodriguez"));

    // Take only strings which match the email regex
    PCollection<String> result =
        emails.apply(Regex.matches("([a-zA-Z0-9._%-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,6})"));
```

## Conclusion

Check out other useful transforms from the [official Apache Beam documentation](https://beam.apache.org/documentation/transforms/java/overview/).