If you need to make changes to how this feature works, such as to add support for other postal code formats, here is a list of the files that you need to look at.
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Zips.pm - contains code for loading the zips.txt file into memory and replying to search queries.  Open-ILS / src / perlmods / lib / OpenILS / Application / Search / Zips.pm
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register.js - This is where patron registration logic is located.  The code that queries the ZIP search service and fills the address is located here.  Open-ILS / web / js / ui / default / actor / user / register.js