Need better regex solution in Ruby - ruby

Question

I have following code:
date_time = Time.now.strftime('%Y%m%d%H%M%S')
name = "builder-#{date_time}" # builder-20150923125450
if some_condition
name.sub!("#{date_time}", "one-#{date_time}") # builder-one-20150923125450
end
Above code is working fine.
But I think it could be better as I feel like I am repeating #{date_time} twice here.
I have heard of regex capture and replace. Can we use it here? If yes, how?

Answer 1

To utilize capturing mechanism, you need to use round brackets round a subpattern that you would like to refer to using a back-reference in the replacement string.
Here is an example:
date_time = Time.now.strftime('%Y%m%d%H%M%S')
name = "builder-#{date_time}"
puts name.sub(/^([^-]*-)/, "\\1one-")
See IDEONE demo
The ^([^-]*-) matches and captures all characters other than - from the beginning of the string (^) and a hyphen, and then we refer to the text with \\1 in the replacement string.
Refer to Use Parentheses for Grouping and Capturing at Regular-Expressions.info for more details.
A more optimal way is using a ternary operator when initializing name variable:
a = 1
date_time = Time.now.strftime('%Y%m%d%H%M%S')
name = "builder-" + (some_condition ? "one-" : "") + "#{date_time}"
IDEONE demo

Answer 2

Strategy one - precalculate the prefix:
date_time = Time.now.strftime('%Y%m%d%H%M%S')
prefix = some_condition ? 'builder-one-' : 'builder-'
name = "#{prefix}#{date_time}"
The string 'builder-' is repeated twice here. Obviously, you can DRY it even more, but it's an overkill IMHO.
Strategy two - use a lookahead:
date_time = Time.now.strftime('%Y%m%d%H%M%S')
name = "builder-#{date_time}"
name.sub!(/(?=#{date_time})/, "one-") if some_condition
Now date_time appears only twice. I wouldn't say it's a great improvement. I wouldn't say there is much of a problem to begin with.

Answer 3

"builder-" + ("one-" if some_condition).to_s + date_time
date_time = "right now"
some_condition = true
"builder-" + ("one-" if some_condition).to_s + date_time
#=> "builder-one-right now"
some_condition = false
"builder-" + ("one-" if some_condition).to_s + date_time
#=> "builder-right now"
Note that:
("one-" if false).to_s #=> nil.to_s => ""