Replace N spaces at the beginning of a line with N characters

I am looking for a regex substitution to transform N white spaces at the beginning of a line to N  . So this text:

list:
  - first

should become:

list:
  - first

I have tried:

str = "list:\n  - first"
str.gsub(/(?<=^) */, "&nbsp;")

which returns:

list:
 - first

which is missing one  . How to improve the substitution to get the desired output?

>Solution :

Use gsub with a callback function:

str = "list:\n  - first"
output = str.gsub(/(?<=^|\n)[ ]+/) {|m| m.gsub(" ", "&nbsp;") }

This prints:

list:
  - first

The pattern (?<=^|\n)[ ]+ captures one or more spaces at the start of a line. This match then gets passed to the callback, which replaces each space, one at a time, with  .