You can use the String Element Reference method which is []
Inside the [] can either be a literal substring, an index, or a regex:
> s='abcdefg'
=> "abcdefg"
> s['a']
=> "a"
> s['z']
=> nil
Since nil is functionally the same as false and any substring returned from [] is true you can use the logic as if you use the method .include?:
0> if s[sub_s]
1> puts "\"#{s}\" has \"#{sub_s}\""
1> else
1* puts "\"#{s}\" does not have \"#{sub_s}\""
1> end
"abcdefg" has "abc"
0> if s[sub_s]
1> puts "\"#{s}\" has \"#{sub_s}\""
1> else
1* puts "\"#{s}\" does not have \"#{sub_s}\""
1> end
"abcdefg" does not have "xyz"
Just make sure you don't confuse an index with a sub string:
> '123456790'[8] # integer is eighth element, or '0'
=> "0" # would test as 'true' in Ruby
> '123456790'['8']
=> nil # correct
You can also use a regex:
> s[/A/i]
=> "a"
> s[/A/]
=> nil