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 $ ...

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 [email protected]\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 ^ $.

According to Pickaxe:

> > ^ > Matches the beginning of a line. > > $ > Matches the end of a line. > > \A > Matches the beginning of the string. > > \z > Matches the end of the string. > > \Z > Matches the end of the string unless the string ends with a "\n", in which case it matches just before the "\n".

So, use \A and lowercase \z. If you use \Z someone could sneak in a newline character. This is not dangerous I think, but might screw up algorithms that assume that there's no whitespace in the string. Depending on your regex and string-length constraints someone could use an invisible name with just a newline character.

JavaScript's implementation of Regex treats \A as a literal 'A' (ref). So watch yourself out there and test.

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.

Difference By Example

1. /^foo$/ matches any of the following, /\Afoo\z/ does not:

   whatever1
   foo
   whatever2

foo
whatever2

whatever1
foo

2. /^foo$/ and /\Afoo\z/ all match the following:

   foo