java regex test

### overview of java regex components

1. **pattern**: a compiled representation of a regular expression.
2. **matcher**: an engine that interprets the pattern and performs match operations on an input string.
3. **patternsyntaxexception**: an unchecked exception thrown to indicate a syntax error in a pattern.

### basic syntax

- **literal characters**: characters that match themselves (e.g., `a`, `b`, `1`, `@`).
- **metacharacters**: special characters that have specific meanings (e.g., `.`, `*`, `+`, `?`, `^`, `$`, `[]`, `()`, `{}`, `|`).
- **character classes**: denoted by brackets `[]` (e.g., `[abc]` matches `a`, `b`, or `c`).
- **quantifiers**: specify the number of occurrences (e.g., `*` means 0 or more, `+` means 1 or more, `{n}` means exactly n).

### commonly used regex patterns

- `^abc`: matches any string that starts with "abc".
- `abc$`: matches any string that ends with "abc".
- `a.b`: matches any string that contains "a", followed by any character, followed by "b".
- `[0-9]`: matches any digit.
- `\d`: matches any digit (equivalent to `[0-9]`).
- `\w`: matches any word character (equivalent to `[a-za-z0-9_]`).
- `\s`: matches any whitespace character (spaces, tabs, etc.).

### example code

below is a simple java program demonstrating how to use regex to test for certain patterns in a string.

### explanation of the code

2. **define a regex pattern**: in this example, we define a simple regex pattern for validating email addresses.
3. **compile the pattern**: we compile our regex pattern into a `pattern` object.
4. **create a matcher**: we create a `matcher` that will check the input string against the compiled pa ...

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