Using random number value generation in vim substitution

gistfile1.txt

BACKGROUND

Given a JSON data file for one Use Case, I wanted to repurpose it 
for two other, similar Use Cases while prototyping.  Actual values 
in the copied files didn't matter; they just needed to be different 
to make the prototype appear different between Use Cases.  

RESEARCH

I knew I wanted to do something like [1] so I did a Google search 
for "vim random number substitute" and found [2].  That didn't work 
for me (something about my syntax of 'eval' and 'system' in command 
mode) so I opted to leverage definitions as found in [3] which I 
modified and put in my .vimrc file.  The regex to the left-facing 
curly bracket could have been \d+ but I opted to be lazy.

SOLUTION 

'<,'>s/"foo":.*}/\=eval("printf('%s', '\"foo\":') . Random() . '}'")/

REFERENCES

[1] : https://gist.github.com/3555497
[2] : http://www.commandlinefu.com/commands/view/6880/map-r-do-insert-random-number-in-vim
[3] : http://vim.1045645.n5.nabble.com/simple-16-bit-random-number-generator-td2843842.html