Replace each pattern in regexp - ruby-on-rails

Question

I'm having some trouble to find the right pattern to get the string I want.
My starting string is :
,,,,C3:,D3,E3,F3,,
I would like to have
C3: [D3,E3,F3]
I would like to replace each starting commas by double space
Replace coma after colon by double space and left square bracket
Replace trailing commas by right square bracket
For now, I tried this :
> a = ",,,,C3:,D3,E3,F3,,"
=> ",,,,C3:,D3,E3,F3,,"
> b = a.gsub(/^,*/, " ").gsub(/(?<=:),/, " [").gsub(/[,]*$/,"" ).gsub(/[ ]*$/, "]")
=> " C3: [D3,E3,F3]"
> b == " C3: [D3,E3,F3]"
=> false
I can't reach to replace each starting comma by a double space to obtain 8 spaces in this case.
Could you help me to find the right regexp and if possible to improve my code, please ?

Answer 1

To replace each starting comma with a double space, you need to use \G operator, i.e. .gsub(/\G,/, ' '). That operator tells the regex engine to match at the start of the string and then after each successful match. So, you only replace each consecutive comma in the beginning of the string with .gsub(/\G,/, ' ').
Then, you can add other replacements:
s.gsub(/\G,/, ' ').sub(/,+\z/, ']').sub(/:,+/, ': [')
See the IDEONE demo
s = ",,,,C3:,D3,E3,F3,,"
puts s.gsub(/\G,/, ' ').sub(/,+\z/, ']').sub(/:,+/, ': [')
Output:
C3: [D3,E3,F3]

Answer 2

To construct the desired string, one needs to know:
the number of leading commas (the size of the string comprised of the leading commas)
the string following the leading commas up to and including the colon
the string between the comma following the colon and two or more commas
It is a simple matter to construct a regex that saves each of these three strings to a capture group:
r = /
(,*) # match leading commas in capture group 1
(.+:) # match up and including colon in capture group 2
, # match comma
(.+) # match any number of any characters in capture group 3
,, # match two commas
/x # extended/free-spacing regex definition mode
",,,,C3:,D3,E3,F3,," =~ r
We can now form the desired string from the contents of the three capture groups:
"#{' '*$1.size}#{$2} [#{$3}]"
#=> " C3: [D3,E3,F3]"