More specifically, \A and \z are "start of string" and "end of string" respectively.

Why not \Z, you ask? \Z will match to the end of the string but ignore a trailing newline.

  irb(main):001:0> !!("hello\n" =~ /\Ahello\z/)
  => false
  irb(main):002:0> !!("hello\n" =~ /\Ahello\Z/)
  => true