Regex - capture multiple groups and combine them multiple times in one string

Question

I need to combine some text using regex, but I'm having a bit of trouble when trying to capture and substitute my string. For example - I need to capture digits from the start, and add them in a substitution to every section closed between ||

I have:

||10||a||ab||abc||

I want:

||10||a10||ab10||abc10||

So I need '10' in capture group 1 and 'a|ab|abc' in capture group 2

I've tried something like this, but it doesn't work for me (captures only one [az] group)

(?=.*\|\|(\d+)\|\|)(?=.*\b([a-z]+\b))

Answer 1

I would achieve this without a complex regular expression. For example, you could do this:

input = "||10||a||ab||abc||"
parts = input.scan(/\w+/)   # => ["10", "a", "ab", "abc"]
parts[1..-1].each { |part| part << parts[0] }   # => ["a10", "ab10", "abc10"]

"||#{parts.join('||')}||"

Answer 2

str = "||10||a||ab||abc||"

first = nil
str.gsub(/(?<=\|\|)[^\|]+/) { |s| first.nil? ? (first = s) : s + first }
  #=> "||10||a10||ab10||abc10||" 

The regular expression reads, "match one or more characters in a pipe immediately following two pipes" ( (?<=\\|\\|) being a positive lookbehind ).