How do I check whether a value in a string is an IP address

Ruby has already the needed Regex in the standard library.
Checkout resolv.

require "resolv"

"192.168.1.1"   =~ Resolv::IPv4::Regex ? true : false #=> true
"192.168.1.500" =~ Resolv::IPv4::Regex ? true : false #=> false

"ff02::1"    =~ Resolv::IPv6::Regex ? true : false #=> true
"ff02::1::1" =~ Resolv::IPv6::Regex ? true : false #=> false

If you like it the short way …

require "resolv"

!!("192.168.1.1"   =~ Resolv::IPv4::Regex) #=> true
!!("192.168.1.500" =~ Resolv::IPv4::Regex) #=> false

!!("ff02::1"    =~ Resolv::IPv6::Regex) #=> true
!!("ff02::1::1" =~ Resolv::IPv6::Regex) #=> false

Have fun!

Update (2018-10-08):

From the comments below i love the very short version:

!!(ip_string =~ Regexp.union([Resolv::IPv4::Regex, Resolv::IPv6::Regex]))

Very elegant with rails (also an answer from below):

validates :ip,
          :format => {
            :with => Regexp.union(Resolv::IPv4::Regex, Resolv::IPv6::Regex)
          }