🧪 Pattern Testing

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^\d{3}-\d{2}-\d{4}$
Social Security Number (US format)
IP Address
IPv4 address validation
^(\+\d{1,3}[- ]?)?\d{10}$
Phone number with optional country code
URL
HTTP/HTTPS URL validation
Strong Password
Min 8 chars, uppercase, lowercase, number, special char
\b\w+(?:\s+\w+)*\b
Extract complete words or phrases

📖 Regex Reference Guide

🔤 Character Classes

Pattern Description Example
. Any character except newline a.c matches "abc", "axc"
\d Any digit (0-9) \d{3} matches "123"
\w Word character (a-z, A-Z, 0-9, _) \w+ matches "hello_123"
\s Whitespace character \s+ matches spaces/tabs
[abc] Any character in brackets [aeiou] matches vowels
[^abc] Any character NOT in brackets [^0-9] matches non-digits
[a-z] Character range [A-Z] matches uppercase

🔢 Quantifiers

Pattern Description Example
* 0 or more ab* matches "a", "ab", "abb"
+ 1 or more ab+ matches "ab", "abb"
? 0 or 1 (optional) colou?r matches "color", "colour"
{n} Exactly n times \d{3} matches exactly 3 digits
{n,} n or more times \d{3,} matches 3+ digits
{n,m} Between n and m times \d{3,5} matches 3-5 digits

⚓ Anchors

Pattern Description Example
^ Start of string/line ^Hello matches "Hello world"
$ End of string/line world$ matches "Hello world"
\b Word boundary \bcat\b matches "cat" not "catch"
\B Non-word boundary \Bcat\B matches "cat" in "locate"

🎯 Groups & Captures

Pattern Description Example
(abc) Capturing group (\d{4})-(\d{2}) captures year/month
(?:abc) Non-capturing group (?:https?://)(.+) groups protocol
(a|b) Alternation (OR) (cat|dog) matches "cat" or "dog"
\1 Backreference to group 1 (\w+)\s+\1 matches repeated words

🏁 Flags

Flag Description Example
g Global (find all matches) Without: finds first match only
i Case insensitive hello matches "Hello", "HELLO"
m Multiline (^ and $ match line breaks) ^start matches line beginnings
s Dot matches newlines .* includes line breaks

🔍 Lookahead & Lookbehind

Pattern Description Example
(?=abc) Positive lookahead \d+(?=px) matches numbers before "px"
(?!abc) Negative lookahead \d+(?!px) matches numbers not before "px"
(?<=abc) Positive lookbehind (?<=\$)\d+ matches numbers after "$"
(? Negative lookbehind (? matches numbers not after "$"

💡 Tips & Best Practices

  • Use \d instead of [0-9] for better readability
  • Escape special characters: \. \+ \* \?
  • Use non-capturing groups (?:) when you don't need the capture
  • Be specific: [a-zA-Z] instead of .* when possible
  • Test edge cases and validate with real data
  • Use word boundaries \b to avoid partial matches
  • Consider performance: avoid nested quantifiers like (a+)+