\	Escape

\^	Beginning of line
\$	End of line
\*	Match any # of previous or none
\+	Match any # of previous
\<	Start of word
\>	End of word
\b	A word boundary
\B	A non-word boundary
\A	The begining of the input
\G	The end of the previous match
\Z	The end of the input but the final terminator, if any
\z	The end of the input
\?	Optional
\i	Case-insensitive matching
\g	Global match

\.	Any one character, except newline
[a-z]	Any lowercase letter
[A-Z]	Any uppercase letter
[A-Za-z]	Any Letter
\d OR [0-9]	Any digit
\D OR [^0-9]	Any non-digit
\s	Any white space character [ \t\n\x0B\f\r]
\S	Any non-white space character [^\s]
\w	A word character [a-zA-Z_0-9]
\W	A non-word character [^\w]

\{n\}	Match exactly n times
\{n,\}	Match at least n times
\{x,y\}	Match at least x times, but no more than y times
*?	Match 0 or more times, but a few times as possible
+?	Match 1 or more times, but a few times as possible
??	Match 0 or 1 times, but as few times as possible
\{x,y\}?	Match at least x times, no more than y times, and as few times as possible
(a|b)	a or b

Ex email pattern: grep "\S\+@\S\+\.[A-Za-z]\{2,6\}" file
OR grep -rIhEo "\b[a-zA-Z0-9.-]+@[a-zA-Z0-9.-]+\.[a-zA-Z0-9.-]+\b" file

Ex URL pattern: grep "https\?://\S*\.[a-z]\+" file