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regex Cheatsheet

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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.

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

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

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"

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"

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"

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)"

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`

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"

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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