What you need are backreferences. Try: 
s/338C(\d{3}C\d{3}C\S{8})/338C$1C/g

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Code is (almost) always untested.
http://www.justicepoetic.net/

In reply to Re: Dealing with Regexes for a specific replacement pattern by jweed
in thread Dealing with Regexes for a specific replacement pattern by gridlock

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