I recently started a new programming interview question and answer series on regular expressions. The first installment at Regular expressions interview questions–Part 1 talks about regular expression character sets, groups, quantifiers, shorthand and gives a full solution to matching a a 10-digit, US phone number, with or without parentheses, hyphens, or dots and optional 3 digit area code. Check it out at http://www.programminginterviews.info/2012/07/regular-expressions-interview.html