a	Matches the exact character "a"
123	Matches the exact case-sensitive sequence "123"
HeLLo	Matches the exact case-sensitive sequence "HeLLo"
\.	Escaped dot — matches a literal "." character
.	Unescaped dot — matches any character (except newline)

[abc]	A single character of: a, b, or c
[^abc]	Any character except: a, b, or c
[a-z]	Any character in range a–z
[^a-z]	Any character not in range a–z
\s	Whitespace (space, \t, \n, \r, \f)
\S	Not whitespace — same as [^\s]
\d	Digit (0–9) — same as [0-9]
\D	Not digit — same as [^0-9]
\w	Word character — same as [a-zA-Z_0-9]
\W	Not word character — same as [^a-zA-Z_0-9]

a*	0 or more
a+	1 or more
a?	0 or 1 (optional)
a{n}	Exactly n times
a{n,}	n or more times
a{m,n}	Between m and n times

a*?	0 or more, as few as possible
a+?	1 or more, as few as possible

^	Start of string/line
$	End of string/line
\b	Word boundary (between \w and \W)
\B	Not a word boundary

(...)	Group — treat as a single unit; captures by index
(a|b)	Alternation — matches either a or b
(?P<name>...)	Named group — access by name, not number
(?:...)	Non-capturing group — groups without storing
\1	Backreference to capture group 1

(?=...)	Positive lookahead
(?!...)	Negative lookahead
(?<=...)	Positive lookbehind
(?<!...)	Negative lookbehind