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楊東霖

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Regex Cheat Sheet: The Only Regex Reference You'll Need in 2026

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Regex Cheat Sheet: The Only Regex Reference You'll Need in 2026

Character Classes

Pattern Matches
. Any character except newline
\d Any digit [0-9]
\D Any non-digit
\w Word character [a-zA-Z0-9_]
\W Non-word character
\s Whitespace (space, tab, newline)
\S Non-whitespace
[abc] Any of a, b, or c
[^abc] Not a, b, or c
[a-z] Any lowercase letter

Quantifiers

Pattern Meaning
* 0 or more
+ 1 or more
? 0 or 1 (optional)
{n} Exactly n
{n,} n or more
{n,m} Between n and m
*? / +? Lazy (minimum) match

Anchors

Pattern Meaning
^ Start of string (or line with m flag)
$ End of string (or line with m flag)
\b Word boundary
\B Non-word boundary

Common JavaScript Patterns 

// Email
/^[^\s@]+@[^\s@]+\.[^\s@]+$/

// URL
/https?:\/\/[^\s]+$/

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

// IPv4
/^(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)$/

// Date YYYY-MM-DD
/^\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])$/
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Lookahead and Lookbehind 

// Positive lookahead: X followed by Y
/(?=Y)X/

// Negative lookahead: X not followed by Y
/(?!Y)X/

// Positive lookbehind: X preceded by Y
/(?<=Y)X/

// Negative lookbehind: X not preceded by Y
/(?<!Y)X/

// Example: match "price" not preceded by "$"
/(?<!\$)\d+(\.\d{2})?/  // captures "49.99" but not "$49.99"
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Flags

Flag Meaning
g Global — find all matches
i Case insensitive
m Multiline — ^ and $ match line boundaries
s Dotall — . matches newlines
u Unicode

Python Example 

import re

# Find all emails
text = "Contact alice@example.com or bob@test.org"
emails = re.findall(r'[\w.+-]+@[\w.-]+', text)

# Validate and extract
pattern = r'^(\d{4})-(0[1-9]|1[0-2])-([0-9]{2})$'
match = re.match(pattern, "2026-03-24")
if match:
    year, month, day = match.groups()
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Common Mistakes

Mistake Correct Pattern
Using .+ when .+? is needed .+ is greedy — may over-match
Forgetting ^ and $ anchors Input might contain extra text around the match
Using \d in [a-z] ranges \d is digits only, not the same as 0-9 in character classes

Testing Your Regex

Always test regex in isolation before using in production. Use a regex tester that shows:

  • All matches highlighted
  • Capture groups separated
  • Step-by-step match explanation

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