Reduce integer from delimited string

Sorry for the bad question title. My brain is broken!!

Problem:

I have a string of integers that can be separated by a comma, hyphen or both. e.g.

str = '3,7-17,21'

I need to take those numbers and reduce them by 1 without losing their position in that string.

When I split, I lose commas and hyphens so their positions change.

Is there a nice way to turn my string into 2,6-16,20?

I started on the following but just ended up in a rabbit hole:

def reduce_hours(hours)
  hours
    .enum_for(:scan, /\d+/)
    .map { Regexp.last_match(0) }
    .map(&:to_i)
    .map { |hour| hour - 1 }
    .then { hours }
end

hours = '3,7-17,21'

It just returns the original string. When I debug, it does match the numbers individually and reduce them by 1. It just doesn’t return those changes to the original string. Am I close with that snippet?

Thank you in advance.

>Solution :

enum_for(:scan, ...), Regexp.last_match(0), then – you’re overthinking it.

You can just use gsub to replace each numeric substring by its predecessor:

str = '3,7-17,21'

str.gsub(/\d+/) { |hour| hour.to_i - 1 }
#=> "2,6-16,20"

The to_i round trip is needed, because strings can’t be decremented directly.

The opposite direction is even easier: (via String#succ)

"2,6-16,20".gsub(/\d+/, &:succ)
#=> "3,7-17,21"