How to insert a space at the beginning of a line in a "for next" code loop (.NET regular expressions)

How to insert a space at the beginning of a line in a "for next" code loop (.NET regular expressions)

I am trying to add spaces at the beginning of lines matched with look aroundsfor
line of code
line of code
line of code
line of code
next


and thisis the output I want to getfor
  line of code
  line of code
  line of code
  line of code
next


Please help me with .NET regular expressions