.
Any character except newline
\w
Word character (a-z, A-Z, 0-9, _)
\W
Non-word character
\d
Digit (0-9)
\D
Non-digit
\s
Whitespace character
\S
Non-whitespace character
[abc]
A single character of: a, b, or c
[^abc]
Any single character except: a, b, or c
[a-z]
Any single character in range a-z
^
Start of string
$
End of string
\b
Word boundary
(a|b)
Match a or b
(...)
Capture group
a?
Zero or one of a
a*
Zero or more of a
a+
One or more of a
a{3}
Exactly 3 of a
a{3,}
3 or more of a
a{3,6}
Between 3 and 6 of a