Regex – Difference between \A \z and ^ $ in Ruby 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 $ …

Best Solution

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\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 ^ $.