Reformat string containing uk postcode using regex

How can I format a string using Javascript to match a regex?

I am using UK postcodes which could match any of the following

N1 3LDEC1A 3ADGU34 8RR

I have the following regex which validates a string correctly, but I am unsure how to use the regex as a mask to format EC1A3AD to EC1A 3AD / GU348RR to GU34 8RR / N13LD to N1 3LD.

My regex is /^[A-Za-z]{1,2}[0-9A-Za-z]{1,2}[ ]?[0-9]{0,1}[A-Za-z]{2}$/

Thank you