Field validation adds an extra tab to each field instance, allowing you to specify validation rules for your field instances.
The following validation rules are currently included:

•Regular expression
•Numeric values (optionally specify min and / or max value)
•Minimum length
•Maximum length
•Minimum number of words
•Maximum number of words
•Plain text (disallow tags)
•Must be empty (Anti-Spam: Hide with CSS)
•Words blacklist
•Minimum number of selections required
•Maximum number of selections allowed
•Exact number of selections required
•Unique
•Match another field(or entity property)
•Specific value(s)
•Require at least one of several fields
•Equal values on multiple fields
•Unique values on multiple fields
•Custom PHP function

Documentation for Regular expression

Lullabot write a doc for this module:Write complex validation rules for your fields, http://www.lullabot.com/articles/module-monday-field-validation