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Regular expressions (regex) are patterns used to match character combinations in strings. This cheatsheet covers common regex syntax with working examples.

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Basic Patterns

• `.` - Match any single character (except newline)

Example: `h.t` matches "hat", "hot", "hit"

• `[abc]` - Match any character in brackets

Example: `[aeiou]` matches any vowel

• `[^abc]` - Match any character NOT in brackets

Example: `[^aeiou]` matches any consonant

• `[a-z]` - Match any character in range

Example: `[0-9]` matches any digit

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Quantifiers

• `*` - Zero or more of preceding element

Example: `ab*c` matches "ac", "abc", "abbc", "abbbc"

• `+` - One or more of preceding element

Example: `ab+c` matches "abc", "abbc" but not "ac"

• `?` - Zero or one of preceding element (optional)

Example: `colou?r` matches both "color" and "colour"

• `{n}` - Exactly n occurrences

Example: `\d{3}` matches exactly 3 digits

• `{n,}` - n or more occurrences

Example: `\d{3,}` matches 3 or more digits

• `{n,m}` - Between n and m occurrences

Example: `\d{2,4}` matches 2 to 4 digits

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Anchors

• `^` - Start of string (or start of line in multiline mode)

Example: `^Hello` matches "Hello" only at the start

• `$` - End of string (or end of line in multiline mode)

Example: `world$` matches "world" only at the end

• `\b` - Word boundary

Example: `\bcat\b` matches "cat" but not "category"

• `\B` - Non-word boundary

Example: `\Bcat\B` matches "category" but not "cat"

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Character Classes

• `\d` - Any digit (equivalent to `[0-9]`)

Example: `\d{4}` matches "2024", "1234"

• `\D` - Any non-digit (equivalent to `[^0-9]`)

Example: `\D+` matches "abc", "xyz"

• `\w` - Any word character (letters, digits, underscore)

Example: `\w+` matches "hello", "test123", "my_var"

• `\W` - Any non-word character

Example: `\W+` matches "!!!", " ", "---"

• `\s` - Any whitespace character (space, tab, newline)

Example: `\s+` matches spaces, tabs

• `\S` - Any non-whitespace character

Example: `\S+` matches "hello", "123"

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Groups and Capturing

• `(abc)` - Capture group

Example: `(hello)` captures "hello" for backreference

• `(?:abc)` - Non-capturing group

Example: `(?:hello|hi)` matches but doesn't capture

• `|` - Alternation (OR)

Example: `cat|dog` matches "cat" or "dog"

• `\1, \2, ...` - Backreferences to captured groups

Example: `(\w)\1` matches "aa", "bb", "11"

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Special Characters (Escaping)

• `\.` - Literal period

Example: `\.com` matches ".com"

• `\+` - Literal plus sign

Example: `\+1` matches "+1"

• `\*` - Literal asterisk

Example: `\*note` matches "*note"

• `\?` - Literal question mark

Example: `really\?` matches "really?"

• `\(` `\)` - Literal parentheses

Example: `\(test\)` matches "(test)"

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Common Patterns

Email Address (Simple)

[\w.-]+@[\w.-]+\.\w+

Matches: `user@example.com`, `test.email@domain.co.uk`

Phone Number (US Format)

\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}

Matches: `(555)123-4567`, `555-123-4567`, `555.123.4567`

URL

https?://[\w.-]+(?:\.[\w.-]+)*(?:/[\w./?=&#-]*)?

Matches: `https://example.com`, `http://site.com/path?q=1`

IP Address

\b(?:\d{1,3}\.){3}\d{1,3}\b

Matches: `192.168.1.1`, `10.0.0.1`

Date (MM/DD/YYYY)

\d{2}/\d{2}/\d{4}

Matches: `12/25/2024`, `01/01/2025`

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Flags/Modifiers (Common)

• `i` - Case-insensitive matching

Example: `/hello/i` matches "Hello", "HELLO", "hello"

• `g` - Global matching (find all, not just first)

Example: `/cat/g` finds all occurrences of "cat"

• `m` - Multiline mode (^ and $ match line boundaries)

Example: `/^line/m` matches "line" at start of any line

• `s` - Dotall mode (. matches newline)

Example: `/hello.world/s` matches "hello\nworld"

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Tips

• Test regex patterns with online tools like regex101.com or regexr.com

• Escape special characters with backslash when you want literal matches

• Use non-greedy quantifiers (`*?`, `+?`) for minimal matches

• Group related patterns with parentheses for better organization

• Use anchors (`^`, `$`) to ensure full string matching

• Different tools/languages may have slightly different regex syntax

• Common uses: validation, search/replace, parsing, extraction

• Start simple and build complexity incrementally

• Document complex regex patterns with comments if your tool supports it

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