# Regular Expressions (Regex)

Regular Expressions (Regex) are sequences of characters that define a search pattern. They are extremely powerful for text processing, validation (like checking emails), and string manipulation.

## 1. Basic Syntax

*   `.` : Matches any single character (except newline).
*   `^` : Anchors to the **start** of the string.
*   `$` : Anchors to the **end** of the string.
*   `|` : Acts as a logical **OR** (e.g., `cat|dog` matches "cat" or "dog").
*   `\` : Escapes special characters (e.g., `\.` matches a literal dot).

## 2. Character Sets & Classes

*   `[abc]` : Matches any one character inside the brackets (a, b, or c).
*   `[^abc]` : Matches any character **NOT** inside the brackets.
*   `[a-z]` : Matches any lowercase letter.

### Shorthand Classes
*   `\d` : Any digit (0-9).
*   `\w` : Any word character (a-z, A-Z, 0-9, _).
*   `\s` : Any whitespace (space, tab, newline).
*   `\D`, `\W`, `\S` : The inverse (non-digit, non-word, non-whitespace).

## 3. Quantifiers

Quantifiers specify how many times the preceding element should match.

*   `*` : 0 or more times.
*   `+` : 1 or more times.
*   `?` : 0 or 1 time (optional).
*   `{3}` : Exactly 3 times.
*   `{2,5}` : Between 2 and 5 times.

## 4. Groups and Capturing

*   `(abc)` : Groups characters together. Also captures the match for later use.
*   `(?:abc)` : Non-capturing group (groups but doesn't save the match).

## 5. Usage in Languages

### JavaScript
In JS, regex literals are written between slashes `/pattern/flags`.

```javascript
const emailPattern = /^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/;
const email = "test@example.com";

if (emailPattern.test(email)) {
    console.log("Valid email");
}

const text = "The year is 2023";
const year = text.match(/\d{4}/); // ["2023"]
```

### Python
Python uses the `re` module.

```python
import re

text = "The price is $99"
match = re.search(r'\$(\d+)', text)

if match:
    print(match.group(1)) # Output: 99
```

[[programming/common-syntax]]