All of the other similar regexp variables ($& $' $+ $~ $(backquote)) are listed by global_variables, so it would make sense for $1 and similar to appear, or alternatively for the other 5 regexp globals not to appear if there is no match. If there is no backref, the other 5 regexp globals still appear. If there is a backref, and it has no captures, it doesn't make sense for $+ to be included and $1 not to be included.
I'm not sure whether $1 through $9 should be present for consistency, even the last match did not have that number of captures. I think that would be more consistent to include all of them if we are including $+ in the case of no matches. However, that would make global_variables less consistent with defined?.
Related to this, defined? is a little strange for the regexp globals:
puts %w[$~ $& $` $' $+ $1 $2].map{|s| "#{s}: #{eval("defined?(#{s})").inspect}"}
# $~: "global-variable"
# $&: nil
# $`: nil
# $': nil
# $+: nil
# $1: nil
# $2: nil
/(.)/ =~ "a"
puts %w[$~ $& $` $' $+ $1 $2].map{|s| "#{s}: #{eval("defined?(#{s})").inspect}"}
# $~: "global-variable"
# $&: "global-variable"
# $`: "global-variable"
# $': "global-variable"
# $+: "global-variable"
# $1: "global-variable"
# $2: nil
I would assume that defined?($~) should be nil if the others are nil. Alternatively, it would make sense for defined? to always return global-variable for $& $' $~ $(backquote). $! is similar to the regexp globals in terms of scoping, and it returns global-variable even if not set.
I checked and defined?($+) is nil if the backref has no captures. I don't think we can change the behavior of defined?($+) or defined?($1) without significantly breaking backwards compatibility.
Also kind of weird is that defined? treats aliases of these global variables differently than it treats the variables themselves:
defined?($&) # nil
alias $MATCH $&
defined?($&) # nil
defined?($MATCH) # "global-variable"
I would think we would want defined? to treat global variable aliases the same as the global variables they alias.
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