Regular eExpression in Salesforce Validation Rules

A regular expression is a string used to describe a format of a string according to certain syntax rules.

[ab]   - A single character, either a or b
[a-zA-Z]  - A single uppercase or lowercase character
\\d         - A digit
\\s         - A whitespace character
X?         - Zero or one occurence of X
X*         - Zero or more occurences of X
X+         - One or More occurences of X
X{n} - n occurences of X

Sample Regular expression validation rules:

http://www.infallibletechie.com/2012/10/regular-expression-for-email-validation.html

http://www.infallibletechie.com/2013/09/regular-expression-for-numbers-alone.html

http://www.infallibletechie.com/2012/06/regular-expression-using-apex-in.html

http://www.infallibletechie.com/2012/11/validation-rule-for-numbers-and-special.html

Cheers!!!