In editor vi (popular on UNIX/Linux), there're lots of nice features, such as search for or replace strings with a regular expression pattern. Is there a plugin that provides these functionalities? Say I want to search for a word that begins with a capital letter followed by 2 or 3 digits and ending with either either x or y. (This is just an example. I don't need this specific search for now.) I know we can write a VBA program to do this search. But if there's a plugin, we don't need to write VBA every time we need this.

Word's wildcard Find & Replace function can do that. For the scenario you describe:
Find = [A-Z][0-9]{2}[xy]
No plugins required. For more complex regular-expression searches outside what a wildcard Find & Replace can do, you can use RegEx, but that needs to be driven programmatically.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thank you, Paul!