Regular Expressions

Anchors
^            Start of string
\A           Start of string
$            End of string
\Z           End of string
\b           Word boundary
\B           Not word boundary
\<           Start of word
\>           End of word

 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

Escape Character
\ Escape Character

Metacharacters (must be escaped)
^ $ ( ) < . * + ? [  \ |  

Quantifiers
*          0 or more
+          1 or more
?          0 or 1
{3}       Exactly 3
{3,}      3 or more
{3,5}    3, 4 or 5

Special Characters
\n    New line
\r     Carriage return
\t     Tab
\v     Vertical tab
\f      Form feed
\xxx  Octal character xxx
\xhh  Hex character hh