楊東霖

Posted on Mar 25 • Originally published at devplaybook.cc

Regex Cheatsheet for Developers: Common Patterns with Examples

#webdev #programming #productivity #devtools

Regular expressions are one of those skills that pays dividends for years. Once you internalize the syntax, you can validate form inputs, parse log files, transform data, and search code — all with a few characters.

This cheatsheet covers the essential regex syntax, the most useful patterns, and practical examples you can use directly. Test all patterns in your browser with the Regex Tester — paste a pattern, test it against real input, and see matches highlighted live.

Core Syntax Reference

Character Matching

Pattern	Matches
`.`	Any character except newline
`\d`	Digit (`0-9`)
`\D`	Non-digit
`\w`	Word character (`[a-zA-Z0-9_]`)
`\W`	Non-word character
`\s`	Whitespace (space, tab, newline)
`\S`	Non-whitespace
`\b`	Word boundary
`\B`	Non-word boundary
`^`	Start of string (or line with `m` flag)
`$`	End of string (or line with `m` flag)
`\n`	Newline
`\t`	Tab

Character Classes

[abc]       # Match a, b, or c
[^abc]      # Match anything except a, b, c
[a-z]       # Lowercase letters
[A-Z]       # Uppercase letters
[0-9]       # Digits (same as \d)
[a-zA-Z]    # All letters
[a-zA-Z0-9] # Alphanumeric (same as \w without _)

Quantifiers

Pattern	Meaning
`*`	0 or more (greedy)
`+`	1 or more (greedy)
`?`	0 or 1 (optional)
`{n}`	Exactly n times
`{n,}`	n or more times
`{n,m}`	Between n and m times
`*?`	0 or more (lazy/non-greedy)
`+?`	1 or more (lazy/non-greedy)

Greedy vs Lazy:

# Input: "<b>bold</b> and <i>italic</i>"

<.+>   # Greedy: matches "<b>bold</b> and <i>italic</i>" (entire string)
<.+?>  # Lazy: matches "<b>", "</b>", "<i>", "</i>" separately

Groups and Alternation

(abc)       # Capturing group — stores match in $1, \1
(?:abc)     # Non-capturing group — groups without storing
(?P<name>abc) # Named group (Python) — access as match.group('name')
(?<name>abc)  # Named group (JS/PCRE)
a|b         # Alternation: match a or b

Lookahead and Lookbehind

foo(?=bar)  # Positive lookahead: "foo" followed by "bar"
foo(?!bar)  # Negative lookahead: "foo" NOT followed by "bar"
(?<=foo)bar # Positive lookbehind: "bar" preceded by "foo"
(?<!foo)bar # Negative lookbehind: "bar" NOT preceded by "foo"

// Lookahead example: price values without the $ sign
const prices = "$10.99, $5.00, free";
const amounts = prices.match(/(?<=\$)\d+\.\d{2}/g);
console.log(amounts); // ['10.99', '5.00']

Flags

Flag	Effect
`i`	Case-insensitive
`g`	Global (find all matches, not just first)
`m`	Multiline (`^` and `$` match line start/end)
`s`	Dotall (`.` matches newlines too)
`u`	Unicode support
`x`	Extended (allow whitespace and comments — Python/Perl)

Regex in JavaScript

// Test if a pattern matches
const pattern = /^\d{5}(-\d{4})?$/;
pattern.test('12345');       // true
pattern.test('12345-6789');  // true
pattern.test('1234');        // false

// Find first match
const match = 'Hello World 2024'.match(/\d+/);
console.log(match[0]);  // '2024'
console.log(match.index);  // 12

// Find all matches (global flag)
const all = 'abc 123 def 456'.match(/\d+/g);
console.log(all);  // ['123', '456']

// Capture groups
const date = '2024-03-15'.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(date[1], date[2], date[3]);  // '2024' '03' '15'

// Named groups
const { groups } = '2024-03-15'.match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
console.log(groups.year, groups.month, groups.day);  // '2024' '03' '15'

// Replace
const clean = 'Hello   World'.replace(/\s+/g, ' ');
console.log(clean);  // 'Hello World'

// Replace with function
const result = 'foo_bar_baz'.replace(/_([a-z])/g, (_, char) => char.toUpperCase());
console.log(result);  // 'fooBarBaz'

// Split
const parts = 'one, two,three ,four'.split(/\s*,\s*/);
console.log(parts);  // ['one', 'two', 'three', 'four']

Regex in Python

import re

# Compile for reuse (faster when using same pattern multiple times)
pattern = re.compile(r'^\d{5}(-\d{4})?$')

# Test match
print(bool(pattern.match('12345')))      # True
print(bool(pattern.match('12345-6789'))) # True
print(bool(pattern.match('1234')))       # False

# Find first match
m = re.search(r'\d+', 'Hello 2024 World')
print(m.group())   # '2024'
print(m.start())   # 6

# Find all matches
all_numbers = re.findall(r'\d+', 'abc 123 def 456')
print(all_numbers)  # ['123', '456']

# Capture groups
m = re.match(r'(\d{4})-(\d{2})-(\d{2})', '2024-03-15')
print(m.groups())  # ('2024', '03', '15')

# Named groups
m = re.match(r'(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})', '2024-03-15')
print(m.group('year'), m.group('month'), m.group('day'))  # 2024 03 15

# Replace
clean = re.sub(r'\s+', ' ', 'Hello   World')
print(clean)  # 'Hello World'

# Replace with function
def to_camel(m):
    return m.group(1).upper()
result = re.sub(r'_([a-z])', to_camel, 'foo_bar_baz')
print(result)  # 'fooBarBaz'

# Split
parts = re.split(r'\s*,\s*', 'one, two,three ,four')
print(parts)  # ['one', 'two', 'three', 'four']

# Flags
pattern = re.compile(r'hello', re.IGNORECASE | re.MULTILINE)

20 Ready-to-Use Patterns

Email Validation

^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$

const emailRegex = /^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$/;
emailRegex.test('user@example.com');  // true
emailRegex.test('user@.com');         // false

URL

https?:\/\/[^\s/$.?#].[^\s]*

IPv4 Address

^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$

Dates (YYYY-MM-DD)

^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12][0-9]|3[01])$

US Phone Number

^(\+1)?[\s.-]?\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}$

Credit Card Number

^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|3[47][0-9]{13}|6(?:011|5[0-9]{2})[0-9]{12})$

Strong Password

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*]).{8,}$

Username (letters, digits, underscore, hyphen, 3-20 chars)

^[a-zA-Z0-9_\-]{3,20}$

Hex Color Code

^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$

ZIP Code (US)

^\d{5}(-\d{4})?$

HTML Tag (opening)

<([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>

JSON String (basic)

"(\\.|[^"\\])*"

Semver Version

^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$

Git Commit Hash (short or full)

\b[0-9a-f]{7,40}\b

JWT Token

^[A-Za-z0-9-_]+\.[A-Za-z0-9-_]+\.[A-Za-z0-9-_]*$

Markdown Link

\[([^\]]+)\]\(([^)]+)\)

Log Timestamp (ISO 8601)

\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d+)?(?:Z|[+-]\d{2}:\d{2})

Slug (URL-friendly string)

^[a-z0-9]+(?:-[a-z0-9]+)*$

Extract Domain from URL

const domainRegex = /^(?:https?:\/\/)?(?:www\.)?([^\/\s]+)/;
const match = 'https://www.example.com/path'.match(domainRegex);
console.log(match[1]); // 'example.com'

Find All TODO Comments

\/\/\s*TODO:?\s*(.+)$

Common Mistakes

Forgetting to escape special characters:

# Wrong: . matches any character
price\.99   # This matches "price.99", "priceX99", etc.

# Right: escape the dot
price\.99   # Only matches "price.99"

Greedy matching when you want lazy:

# Input: <span>text</span>
<.+>   # Greedy: matches the whole string
<.+?>  # Lazy: matches "<span>" and "</span>" separately

Anchoring issues:

# Without anchors: matches anywhere in string
\d+   # Matches "1" in "abc123def"

# With anchors: must match full string
^\d+$  # Only matches strings that are entirely digits

Catastrophic backtracking:

# Dangerous pattern on long strings with no match:
(a+)+b   # Exponential time complexity

# Safe alternative using possessive/atomic (where supported):
(?>a+)+b
# Or rewrite to avoid nested quantifiers

Test all these patterns and build new ones with the Regex Tester — it highlights matches, shows capture groups, and explains what each part of your expression does.

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