How to replace string using rex with partial matched string?

[LearningGuy]

How to replace string using rex with partial matched string?
Thank you for your help.
For example:
I tried to replace "::" (double colon) with ":0:"  (colon zero colon) if it the previous characters contain ":" followed by "1 to 3 characters" 
| rex mode=sed field=ip "s/:.{1,3}::/:.{1,3}:0:/g"   => this does not work because it will literally replace it with
":.{1,3}:0:"   instead of retaining the matched strings

Before
a0:1::21
b0:1c::21
c0:a13::23

After
a0:1:0:21
b0:11:0:21
c0:111:0:23

[LearningGuy]

It worked!!  Thank you..    

[richgalloway · Accepted solution]

Characters to be retained should be enclosed in a capture group and that group referenced in the replacement text.

| rex mode=sed field=ip "s/:(.{1,3})::/:\1:0:/g"