[abc]- A single character of: a, b, or c
[^abc]- Any single character except: a, b, or c
[a-z]- Any single character in the range a-z
[a-zA-Z]- Any single character in the range a-z or A-Z
^- Start of line
$- End of line
\A- Start of string
\z- End of string
.- Any single character
\s- Any whitespace character
\S- Any non-whitespace character
\d- Any digit
\D- Any non-digit
\w- Any word character (letter, number, underscore)
\W- Any non-word character
\b- Any word boundary
(...)- Capture everything enclosed
(a|b)- a or b
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