Regex Cheatsheet

.        Any character (except newline)
\d       Digit (0-9)
\D       Non-digit
\w       Word character (a-z, A-Z, 0-9, _)
\W       Non-word character
\s       Whitespace (space, tab, newline)
\S       Non-whitespace
\b       Word boundary
^        Start of string
$        End of string

*        0 or more
+        1 or more
?        0 or 1 (optional)
{3}      Exactly 3
{2,5}    Between 2 and 5
{2,}     2 or more

(abc)       Capture group
(?:abc)     Non-capturing group
a|b         Alternation (a or b)
(?=abc)     Positive lookahead
(?!abc)     Negative lookahead
(?<=abc)    Positive lookbehind
(?<!abc)    Negative lookbehind

Email:        /^[^\s@]+@[^\s@]+\.[^\s@]+$/
URL:          /^https?:\/\/.+/
Numbers:      /^\d+$/
Username:     /^[a-zA-Z0-9_]{3,16}$/
Hex color:    /^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/
IPv4:         /^(\d{1,3}\.){3}\d{1,3}$/
Date:         /^\d{4}-\d{2}-\d{2}$/

g   Global — find all matches
i   Case insensitive
m   Multiline (^ and $ match line start/end)
s   Dotall (. matches newline too)