if your inputfield is an sObject field you can use a validation rule with a regex expression (REGEX( Social_Security_Number__c, "[0-9]{3}-[0-9]{2}-[0-9]{4}") ) for instance
if not an sObject field you can do it in apex, for instance :
String InputString = 'email@email.com';
String emailRegex = '([a-zA-Z0-9_\\-\\.]+)@((\\[a-z]{1,3}\\.[a-z]{1,3}\\.[a-z]{1,3}\\.)|(([a-zA-Z0-9\\-]+\\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})';
Pattern MyPattern = Pattern.compile(emailRegex);
// Then instantiate a new Matcher object "MyMatcher"
Matcher MyMatcher = MyPattern.matcher(InputString);
if (!MyMatcher.matches()) {
// invalid, do something
}
if your inputfield is an sObject field you can use a validation rule with a regex expression (REGEX( Social_Security_Number__c , "[0-9]{3}-[0-9]{2}-[0-9]{4}") ) for instance
if not an sObject field you can do it in apex, for instance :
source: http://salesforcesource.blogspot.be/2010/01/utilizing-apex-pattern-and-matcher.html
email validation regular expressions should be abundant to find with a search engine.