Construct 
Matches 

Characters 
x 
The character x 
\\ 
The backslash character 
\0n 
The character with octal value 0n
(0 <= n <= 7) 
\0nn 
The character with octal value 0nn
(0 <= n <= 7) 
\0mnn 
The character with octal value 0mnn
(0 <= m <= 3,
0 <= n <= 7) 
\xhh 
The character with hexadecimal value 0xhh 
\uhhhh 
The character with hexadecimal value 0xhhhh 
\t 
The tab character ('\u0009') 
\n 
The newline (line feed) character ('\u000A') 
\r 
The carriagereturn character ('\u000D') 
\f 
The formfeed character ('\u000C') 
\a 
The alert (bell) character ('\u0007') 
\e 
The escape character ('\u001B') 
\cx 
The control character corresponding to x 

Character classes 
[abc] 
a, b, or c (simple class) 
[^abc] 
Any character except a, b, or c (negation) 
[azAZ] 
a through z
or A through Z, inclusive (range) 
[ad[mp]] 
a through d,
or m through p: [admp] (union) 
[az&&[def]] 
d, e, or f (intersection) 
[az&&[^bc]] 
a through z,
except for b and c: [adz] (subtraction) 
[az&&[^mp]] 
a through z,
and not m through p: [alqz](subtraction) 

Predefined character classes 
. 
Any character (may or may not match line terminators) 
\d 
A digit: [09] 
\D 
A nondigit: [^09] 
\s 
A whitespace character: [ \t\n\x0B\f\r] 
\S 
A nonwhitespace character: [^\s] 
\w 
A word character: [azAZ_09] 
\W 
A nonword character: [^\w] 

POSIX character classes (USASCII only) 
\p{Lower} 
A lowercase alphabetic character: [az] 
\p{Upper} 
An uppercase alphabetic character:[AZ] 
\p{ASCII} 
All ASCII:[\x00\x7F] 
\p{Alpha} 
An alphabetic character:[\p{Lower}\p{Upper}] 
\p{Digit} 
A decimal digit: [09] 
\p{Alnum} 
An alphanumeric character:[\p{Alpha}\p{Digit}] 
\p{Punct} 
Punctuation: One of !"#$%&'()*+,./:;<=>?@[\]^_`{}~ 
\p{Graph} 
A visible character: [\p{Alnum}\p{Punct}] 
\p{Print} 
A printable character: [\p{Graph}] 
\p{Blank} 
A space or a tab: [ \t] 
\p{Cntrl} 
A control character: [\x00\x1F\x7F] 
\p{XDigit} 
A hexadecimal digit: [09afAF] 
\p{Space} 
A whitespace character: [ \t\n\x0B\f\r] 

Classes for Unicode blocks and categories 
\p{InGreek} 
A character in the Greek block (simple block) 
\p{Lu} 
An uppercase letter (simple category) 
\p{Sc} 
A currency symbol 
\P{InGreek} 
Any character except one in the Greek block (negation) 
[\p{L}&&[^\p{Lu}]] 
Any letter except an uppercase letter (subtraction) 

Boundary matchers 
^ 
The beginning of a line 
$ 
The end of a line 
\b 
A word boundary 
\B 
A nonword boundary 
\A 
The beginning of the input 
\G 
The end of the previous match 
\Z 
The end of the input but for the final
terminator, if any 
\z 
The end of the input 

Greedy quantifiers 
X? 
X, once or not at all 
X* 
X, zero or more times 
X+ 
X, one or more times 
X{n} 
X, exactly n times 
X{n,} 
X, at least n times 
X{n,m} 
X, at least n but not more than m times 

Reluctant quantifiers 
X?? 
X, once or not at all 
X*? 
X, zero or more times 
X+? 
X, one or more times 
X{n}? 
X, exactly n times 
X{n,}? 
X, at least n times 
X{n,m}? 
X, at least n but not more than m times 

Possessive quantifiers 
X?+ 
X, once or not at all 
X*+ 
X, zero or more times 
X++ 
X, one or more times 
X{n}+ 
X, exactly n times 
X{n,}+ 
X, at least n times 
X{n,m}+ 
X, at least n but not more than m times 

Logical operators 
XY 
X followed by Y 
XY 
Either X or Y 

Grouping operators 
(X) 
X. This capturing group is remembered so it can be referenced later. Numbered starting at 1. 
(?:X) 
X. Used purely for grouping, and isn't remembered. 

Back references 
\n 
Whatever the n^{th}
capturing group matched 

Quotation 
\ 
Nothing, but quotes the following character 
\Q 
Nothing, but quotes all characters until \E 
\E 
Nothing, but ends quoting started by \Q 

Special constructs (all groupings are noncapturing) 
(?idmsuxidmsux) 
Nothing, but turns match flags on  off 
(?idmsuxidmsux:X) 
X, as a noncapturing group with the
given flags on  off 
(?=X) 
X, via zerowidth positive lookahead 
(?!X) 
X, via zerowidth negative lookahead 
(?<=X) 
X, via zerowidth positive lookbehind 
(?<!X) 
X, via zerowidth negative lookbehind 
(?>X) 
X, as an independent, noncapturing group 