regex-patterns

# Regex Patterns

Practical regular expression cookbook. Patterns for validation, parsing, extraction, and refactoring across JavaScript, Python, Go, and command-line tools.

## When to Use

- Validating user input (email, URL, IP, phone, dates)
- Parsing log lines or structured text
- Extracting data from strings (IDs, numbers, tokens)
- Search-and-replace in code (rename variables, update imports)
- Filtering lines in files or command output
- Debugging regexes that don't match as expected

## Quick Reference

### Metacharacters

| Pattern | Matches | Example |
|---|---|---|
| `.` | Any character (except newline) | `a.c` matches `abc`, `a1c` |
| `\d` | Digit `[0-9]` | `\d{3}` matches `123` |
| `\w` | Word char `[a-zA-Z0-9_]` | `\w+` matches `hello_123` |
| `\s` | Whitespace `[ \t\n\r\f]` | `\s+` matches spaces/tabs |
| `\b` | Word boundary | `\bcat\b` matches `cat` not `scatter` |
| `^` | Start of line | `^Error` matches line starting with Error |
| `$` | End of line | `\.js$` matches line ending with .js |
| `\D`, `\W`, `\S` | Negated: non-digit, non-word, non-space | |

### Quantifiers

| Pattern | Meaning |
|---|---|
| `*` | 0 or more (greedy) |
| `+` | 1 or more (greedy) |
| `?` | 0 or 1 (optional) |
| `{3}` | Exactly 3 |
| `{2,5}` | Between 2 and 5 |
| `{3,}` | 3 or more |
| `*?`, `+?` | Lazy (match as few as possible) |

### Groups and Alternation

| Pattern | Meaning |
|---|---|
| `(abc)` | Capture group |
| `(?:abc)` | Non-capturing group |
| `(?P<name>abc)` | Named group (Python) |
| `(?<name>abc)` | Named group (JS/Go) |
| `a\|b` | Alternation (a or b) |
| `[abc]` | Character class (a, b, or c) |
| `[^abc]` | Negated class (not a, b, or c) |
| `[a-z]` | Range |

### Lookahead and Lookbehind

| Pattern | Meaning |
|---|---|
| `(?=abc)` | Positive lookahead (followed by abc) |
| `(?!abc)` | Negative lookahead (not followed by abc) |
| `(?<=abc)` | Positive lookbehind (preceded by abc) |
| `(?<!abc)` | Negative lookbehind (not preceded by abc) |

## Validat...