Difference between \A \Z and ^ $ in Ruby on Rails regular expressions

In the documentation I read:

use \A and \Z to match the start and end of the string, ^ and $ match the start/end of a line

I am going to apply a regular expression to check username (or e-mail is the same) submitted by user. Which expression should I use with validates_format_of in model? I can't understand the difference: I've always used ^ and $ ...

From stackoverflow

• If you're depending on the regular expression for validation, you always want to use \A and \Z. ^ and $ will only match up until a newline character, which means they could use an email like me@example.com\n<script>dangerous_stuff();</script> and still have it validate, since the regex only sees everything before the \n.

  My recommendation would just be completely stripping new lines from a username or email beforehand, since there's pretty much no legitimate reason for one. Then you can safely use EITHER \A \Z or ^ $.

• The start and end of a string may not necessarily be the same thing as the start and end of a line. Imagine if you used the following as your test string:

  my
  name
  is
  Andrew

  Notice that the string has many lines in it - the ^ and $ characters allow you to match the beginning and end of those lines (basically treating the \n character as a delimeter) while \A and \Z allow you to match the beginning and end of the entire string.