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en/tutorials/js/33js-regular-expressions

# JavaScript Regular Expressions

Match patterns in strings.

## Creating Regex

```javascript
let pattern = /hello/;           // Literal notation
let pattern = new RegExp("hello"); // Constructor (for dynamic patterns)
let pattern = /hello/gi;         // With flags
```

## Basic Methods

```javascript
let pattern = /hello/i;

pattern.test("Hello World");      // true (boolean)
"Hello World".match(pattern);     // ["Hello"]
"Hello World".search(pattern);    // 0 (index)
"Hello World".replace(pattern, "Hi"); // "Hi World"
```

## Character Classes

```javascript
/\d+/.test("123");    // true (digits)
/\w+/.test("hello");  // true (word characters)
/\s+/.test(" ");      // true (whitespace)
/.llo/.test("hello"); // true (any character)
```

Uppercase variants match inverses: `\D` (non-digit), `\W` (non-word), `\S` (non-whitespace).

## Quantifiers

```javascript
/ab*/.test("a");      // true (zero or more b's)
/ab+/.test("a");      // false (one or more b's)
/ab?/.test("a");      // true (zero or one b)
/a{2,4}/.test("aaa"); // true (2 to 4 a's)
```

## Anchors

```javascript
/^hello/.test("hello world");  // true (start of string)
/hello$/.test("say hello");    // false (not at end)
/^hello$/.test("hello");       // true (exact match)
```

**Remember:** Use `^` and `$` for validation to ensure the entire string matches.

## Groups and Alternation

```javascript
/(cat|dog)/.test("I have a cat"); // true (OR)

let match = "123-456-7890".match(/(\d{3})-(\d{3})-(\d{4})/);
console.log(match[1]); // "123" (capturing group)
```

## Flags

| Flag | Meaning |
|------|---------|
| `g` | Global (all matches) |
| `i` | Case-insensitive |
| `m` | Multiline (`^`/`$` match lines) |
| `s` | Dotall (`.` matches newlines) |

## Email Validation

```javascript
let emailPattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
emailPattern.test("test@example.com"); // true
emailPattern.test("invalid");          // false
```

## Phone Validation

```javascript
let phonePattern = /^\+?[\d\s-]{10,}$/;
phonePattern.test("+1-555-123-4567"); // true
phonePattern.test("123");              // false
```

## Password Validation

```javascript
let passPattern = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/;
passPattern.test("MyP@ss123");  // true
passPattern.test("weak");        // false
```

Uses lookaheads `(?=...)` to require lowercase, uppercase, digit, and min 8 chars.

## Extracting Data

```javascript
let text = "Date: 2024-01-15";
let match = text.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(match[1], match[2], match[3]); // "2024" "01" "15"
```

## Best Practices

- **Test thoroughly** — Edge cases matter
- **Use character classes** — More readable
- **Use anchors** — For exact matching
- **Keep patterns simple** — Document complex ones

## Common Mistakes

- **Forgetting `^` and `$`** — Partial matches
- **Not using `g` flag** — Only finds first match
- **Overcomplicating** — Keep patterns simple
- **Not escaping special chars** — `. * + ? ( ) [ ] { } \ ^ $ |`

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