Ruby special variables

There is a collection of special variables whose names consist of a dollar sign ($) followed by a single character. For example, $$ contains the process id of the ruby interpreter, and is read-only.

$	global variable
@	instance variable
[a-z] or _	local variable
[A-Z]	constant

Here are the major system variables:

$!	latest error message
$@	location of error
$_	string last read by gets
$.	line number last read by interpreter
$&	string last matched by regexp
$~	the last regexp match, as an array of subexpressions
$n	the nth subexpression in the last match (same as $~[n])
$=	case-insensitivity flag
$/	input record separator
$\	output record separator
$0	the name of the ruby script file
$*	the command line arguments
$$	interpreter's process ID
$?	exit status of last executed child process

In the above, $_ and $~ have local scope. Their names suggest they should be global, but they are much more useful this way, and there are historical reasons for using these names.