# Regular Expressions Quick Reference

*Patterns, quantifiers, groups, lookaheads, flags*

> Source: Regular Expressions Reference (regular-expressions.info) · MIT

## Basic Patterns

### Metacharacters

| Command | Description |
|---------|-------------|
| `.` | Any character (except newline) |
| `^` | Start of string / line |
| `$` | End of string / line |
| `*` | 0 or more of previous |
| `+` | 1 or more of previous |
| `?` | 0 or 1 of previous (optional) |
| `\` | Escape metacharacter |

### Literal Matching

```
hello     # matches "hello" exactly
a.c       # matches "abc", "a1c", "a-c", etc.
\.txt     # matches literal ".txt"
```

## Character Classes

### Bracket Expressions

| Command | Description |
|---------|-------------|
| `[abc]` | Match a, b, or c |
| `[^abc]` | Match anything except a, b, c |
| `[a-z]` | Lowercase letter |
| `[A-Z]` | Uppercase letter |
| `[0-9]` | Digit |
| `[a-zA-Z0-9]` | Alphanumeric |

### Shorthand Classes

| Command | Description |
|---------|-------------|
| `\d` | Digit `[0-9]` |
| `\D` | Non-digit `[^0-9]` |
| `\w` | Word char `[a-zA-Z0-9_]` |
| `\W` | Non-word char |
| `\s` | Whitespace `[ \t\n\r\f]` |
| `\S` | Non-whitespace |

## Quantifiers

### Greedy Quantifiers

| Command | Description |
|---------|-------------|
| `*` | 0 or more (greedy) |
| `+` | 1 or more (greedy) |
| `?` | 0 or 1 (greedy) |
| `{n}` | Exactly n times |
| `{n,}` | n or more times |
| `{n,m}` | Between n and m times |

### Lazy Quantifiers

| Command | Description |
|---------|-------------|
| `*?` | 0 or more (lazy / non-greedy) |
| `+?` | 1 or more (lazy) |
| `??` | 0 or 1 (lazy) |
| `{n,m}?` | Between n and m (lazy) |

*Lazy quantifiers match as few characters as possible*

### Greedy vs Lazy

```
<.+>      # greedy: "<b>bold</b>"
<.+?>     # lazy:   "<b>"
```

## Anchors

| Command | Description |
|---------|-------------|
| `^` | Start of string (or line with `m` flag) |
| `$` | End of string (or line with `m` flag) |
| `\b` | Word boundary |
| `\B` | Non-word boundary |
| `\A` | Start of string (not affected by `m`) |
| `\Z` | End of string (not affected by `m`) |

### Anchor Examples

```
^Hello        # starts with "Hello"
world$        # ends with "world"
\bword\b      # "word" as whole word
\Bword\B      # "word" inside another word
```

## Groups & Alternation

### Capturing Groups

```
(abc)         # capture group: match "abc"
(a|b|c)       # alternation: a or b or c
(cat|dog)     # match "cat" or "dog"
(\d{3})-(\d{4})  # groups: "123-4567"
```

### Group Types

| Command | Description |
|---------|-------------|
| `(pattern)` | Capturing group |
| `(?:pattern)` | Non-capturing group |
| `(?P<name>pat)` | Named group (Python) |
| `(?<name>pat)` | Named group (JS, .NET) |
| `\1  \2` | Backreference to group 1, 2 |
| `a\|b` | Alternation: a or b |

## Lookahead & Lookbehind

| Command | Description |
|---------|-------------|
| `(?=pattern)` | Positive lookahead |
| `(?!pattern)` | Negative lookahead |
| `(?<=pattern)` | Positive lookbehind |
| `(?<!pattern)` | Negative lookbehind |

### Lookaround Examples

```
\d+(?= USD)     # digits followed by " USD"
\d+(?! USD)     # digits NOT followed by " USD"
(?<=\$)\d+      # digits preceded by "$"
(?<!\$)\d+      # digits NOT preceded by "$"
```

*Lookarounds match a position without consuming characters*

## Common Patterns

| Command | Description |
|---------|-------------|
| `\d{1,3}(\.\d{1,3}){3}` | IPv4 address (basic) |
| `[\w.+-]+@[\w-]+\.[\w.]+` | Email (basic) |
| `https?://[\w./\-?&#=]+` | URL (basic) |
| `\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}` | US phone number |
| `\d{4}-\d{2}-\d{2}` | Date (YYYY-MM-DD) |
| `#?[0-9a-fA-F]{6}` | Hex color code |

*These are simplified patterns; production use may need stricter validation*

## Flags

| Command | Description |
|---------|-------------|
| `g` | Global: find all matches, not just first |
| `i` | Case-insensitive matching |
| `m` | Multiline: `^` / `$` match line boundaries |
| `s` | Dotall: `.` matches newline too |
| `x` | Verbose: ignore whitespace, allow comments |
| `u` | Unicode: full Unicode support |

### Flag Usage by Language

```
/pattern/gi           # JavaScript
re.compile(r"pat", re.I | re.M)  # Python
grep -iE "pattern"    # grep (extended)
```