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Instead of changing the search term directly, you can silently search for both.

Alternatively, you can use the Levenshtein distance to find words close to the search term (like its singular form).

Have you looked into using stemming?

https://en.wikipedia.org/wiki/Stemming

Most major languages should have a library available to handle this for you.

stemming will be baked in to a search engine, for example elasticsearch or solr dependent on the language analyzer you use and how you map your fields.