1. Anchors
=============================================================================
^		Start of string, or start of multi-line pattern
\A		Start of string
$		End of string, or end of multi-line pattern
\Z		End of string
\b		Word boundary
\B		Not word boundary
\<		Start of word
\>		End of word

2. Character Classes
=============================================================================
\c		Control character
\s		White space
\S		Not white space
\d		Digit
\D		Not digit
\w		Word
\W		Not word
\x		Hexadecimal digit
\O		Octal digit

3. Posix
=============================================================================
[:upper:]	Upper case letters
[:lower:]	Lower case letters
[:alpha:]	All letters
[:alnum:]	Digits & letters
[:digit:]	Digits
[:xdigit:]	Hexadecimal digits
[:punct:]	punctuation
[:blank:]	space and tab
[:cntrl:]	Control characters
[:graph:]	Printed characters
[:print:]	Printed characters
[:word:]	Digits, letters and underscore

4. Assertions
=============================================================================
?=		Lookahead assertion
?!		Negative lookahead
?<=		Lookbehind assertion
?!= or ?<!		   Negative lookbehind
?>	    Once-only subexpression
?()		Condition [if then]
?()|	Condition [if then else]
?#		Comment

5. Quantifiers
=============================================================================
*		0 or more
+		1 or more
?		0 or 1
{3}		Exactly 3
{3,}	3 or more
{3,5	3, 4 or 5
add a ? to a quantifier to make it ungreedy

6. Escape Sequences
=============================================================================
\		Escape following characters
\Q		Begin literal sequence
\E		End literal sequence
"Escaping" is a way of treating characters which have a special meaning in
regex literaly, rather than as special characters.

7. Common metacharacters
=============================================================================
^
[
.
$
{
*
(
)
\
+
|
?
<
>
The escape charater is usually \

8. Special Characters
=============================================================================
\n		Newline
\r		Carriage return
\t		Tab
\v		Vertical tab
\f		Form feed
\xxx	Octal character xxx
\xhh	Hex character hh


9. Groups and ranges
=============================================================================
.		Any character exept newline (\n)
(a|b)	A or b
(...)	Group
(?:...)	Passive (non-capturing) group
[abc]	Range (a or b or c)
[^abc] 	not range (not a or b or c)
[a-q] 	lower case letter a to q
[A-Q]	upper case letter a to q
[0-7]	Digit 0-7
\x		Group/subpattern number x
Ranges are inclusive

10. Pattern modifiers
=============================================================================
g		Global match
i *		case-insensitive
m *		multiple lines
s *		Treat string as single line
x *		allow comments and whitespace in pattern
e * 	Evaluate replacement
U *		ungreedy pattern
(* = PCRE modifier)

11. String Replacement
=============================================================================
$n		nth non-passive group
$2		"xyz" in /^(abc(xyz))$/
$1		"xyz" in /^(?:abc)(xyz)$/
$`		Before matched string
$'		After matced string
$+		Last matched string
$&		Entire matched string
(Some regex implementations use \ instead of $