PHP regular expressions explained and examples

php regular expressions summary:	Explanation:
^	Start of String
$	End of string
n*	Zero or more of 'n'
n+	One or more of 'n'
n?	An optional 'n'
n{3}	Exactly three of 'n'
n{3,}	At least 3 or more of 'n'
n{3,6}	From 3 to 6 of 'n'
()	Parenthesis to group expressions
(a\|z)	Either 'a' or 'z'
.	Any single character
\\.	The period '.'
\\\	The backslash '\'
[1-9]	A number between 1 and 9
[a-z]	A lower case character between a and z
[A-Z]	An upper case character between A and Z
[^a-z]	Absence of lower case a to z
[_a-zA-Z]	An underscore or any letter of the alphabet
^([a-zA-Z0-9_]\|\\-\|\\.)+@(([a-zA-Z0-9_]\|\\-)+\\.)+[a-zA-Z]+$	Ex: Used to verify valid e-mail address field format A string beginning with any alphabet (a-z, A-Z) or number (0-9) or underscore or hyphen or period (user-name field) followed by the @ sign followed by any alphabet (a-z, A-Z) or number (0-9) or underscore or hyphen (domain-name field) followed by a period followed by any alphabet (a-z, A-Z) (top-level-domain field) end of string
^([a-zA-Z0-9_]\|\\-\|\\.)+@(([a-zA-Z0-9_]\|\\-)+\\.)+[a-zA-Z]{2,4}$	Ex: e-mail address field format for 2-4 character top-level-domain name This would work for top-level-domain names such as .us, .uk, .com, .info, .name, .coop, etc. But with the new top-level-domain name such as .museum, .family, it won't work.
^.{3}[a-z]{1,3}_?[0-9]*([2-7]\|[a-f])[^1-9]{2}a+$	A string beginning with any three characters followed by from 1 or 3 lower case alphabet letters followed by an optional underscore followed by zero or more digits of 0-9 followed by either a number between 2 and 7 or a character between a and f (lowercase) followed by a two characters which are not digits between 1 and 9 followed by one or more characters of a end of a string

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