java regex tester

### overview of java regex

1. **key classes**:
- `pattern`: represents a compiled representation of a regex.
- `matcher`: used to match a character sequence against a pattern.
- `patternsyntaxexception`: thrown to indicate a syntax error in a regex pattern.

### basic steps to use regex in java

1. **compile a regex pattern**: create a `pattern` object from a regex string.
2. **create a matcher**: use the `pattern` object to create a `matcher` for a specific input string.
3. **perform matching operations**: use methods on the `matcher` object to find matches, replace text, etc.

### example code

here’s a simple example demonstrating how to use regex in java to test if a string matches a specific pattern:

### explanation of the code

1. **pattern**: the regex used here is a basic pattern for validating email addresses.
- `^[a-za-z0-9._%+-]+` - matches the beginning of the email, which can include letters, numbers, and some special characters.
- `@[a-za-z0-9.-]+` - matches the domain name after the "@" symbol.
- `\\.[a-za-z]{2,}$` - ensures that the domain ends with a dot followed by at least two letters (like `.com`, `.org`, etc.).

5. **output**: the program prints whether each email is valid or invalid based on the regex pattern.

### common regex methods

- `matches()`: checks if the entire input string matches ...

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