def valid_v6?(addr)
return true if /\A[\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*\Z/ =~ addr
return true if /\A[\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*::([\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*)?\Z/ =~ addr
return true if /\A::([\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*)?\Z/ =~ addr
return true if /\A[\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*:/ =~ addr && valid_v4?($')
return true if /\A[\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*::([\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*:)?/ =~ addr && valid_v4?($')
return true if /\A::([\dA-Fa-f]{1,4}(:[\dA-Fa-f]{1,4})*:)?/ =~ addr && valid_v4?($')
false
end