Regex PCRE2IntermediateRegex
Regular expression syntax, character classes, groups, lookaheads, and common patterns.
Syntax
2 topics
Character Classes & Quantifiers
# Character classes
. Any character except newline
\d Digit [0-9]
\D Non-digit
\w Word char [a-zA-Z0-9_]
\W Non-word char
\s Whitespace
\S Non-whitespace
[abc] a, b, or c
[^abc] Not a, b, or c
[a-z] Lowercase letter
# Quantifiers
* 0 or more (greedy)
+ 1 or more (greedy)
? 0 or 1
{3} Exactly 3
{3,} 3 or more
{3,6} Between 3 and 6
*? +? ?? Non-greedy variants
# Anchors
^ Start of string/line
$ End of string/line
\b Word boundary💡 Use non-greedy *? when you want the shortest match
⚡ Word boundaries \b are great for whole-word matching
Groups & Lookaheads
# Groups
(abc) Capture group
(?:abc) Non-capturing group
(?P<name>…) Named capture group
# Alternation
cat|dog Matches cat or dog
(jpg|png|gif) Image extensions
# Lookahead / lookbehind
(?=…) Positive lookahead
(?!…) Negative lookahead
(?<=…) Positive lookbehind
(?<!…) Negative lookbehind
# Example: price followed by USD
\d+(?= USD) Matches the number in '42 USD'
# Example: not preceded by $
(?<!\$)\d+ Matches bare numbers💡 Non-capturing groups (?:) are faster — use them when you don't need the match
⚡ Lookaheads/lookbehinds match without consuming characters
Common Patterns
1 topic
Useful Regex Recipes
# Email (basic)
^[\w.+-]+@[\w-]+\.[\w.-]+$
# URL
https?://[\w/:%#$&?()~.=+\-]+
# IPv4
^((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)$
# Date YYYY-MM-DD
^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$
# Hex color
^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$
# Slug (URL-safe)
^[a-z0-9]+(?:-[a-z0-9]+)*$
# Strong password (8+ chars, upper, lower, digit, special)
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$⚠️ Don't validate emails with regex in production — use a library
💡 Test regex at regex101.com with real data before shipping
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