How to replace leading spaces (only) with " " in Vim?

I’ve had this problem many times: I have a piece of source code, but if I copy and paste it into Wordpress and enclose it with the <code>...</code> tags, the beginning spaces are “compressed” into one.

Thus I’d like to know how I could change all the spaces only at the beginning of th开发者_如何学Pythone line by  , so that, for example,

    extend: 'Ext.panel.Panel',

becomes

    extend: 'Ext.panel.Panel',

---

:%s/^ \+/\=repeat(" ",strlen(submatch(0)))

But it wouldn't surprise me if there's a shorter substitute command. Come on Vimgolfers!

---

There are three approaches to implement the desired edit that I can see, listed below in the order of my personal preference.

1. A substitution using the preceding-atom matching syntax (see :help \@<=):

    :%s/\%(^ *\)\@<= /\&nbsp;/g
   

   If brevity of the command is crucial, one can shorten it using the “very magic” mode (see :help \v) by changing the non-capturing group (:help \%() to a capturing one:

    :%s/\v(^ *)@<= /\&nbsp;/g
   

2. A two-staged substitution that splits a line just after the leading spaces, replaces those spaces, and rejoins that line:

    :g/^/s/^ \+/&\r/|-s/ /\ /g|j!
   

3. Another two-step substitution that replaces each of the leading spaces by certain symbol that does not occur in the text, and changes that symbol to  :

    :exe "g/^ \\+/norm!v//e\rr\r" | %s/\r/\ /g
   

---

Using a look-behind assertion to replace spaces precedeed by only spaces at the beginning of a line:

%s/\(^ *\)\@<= /\&nbsp;/g