Best practice for: ERROR: invalid byte sequence for encoding "UTF8"

Hello,

Imagine a web application that process text search queries from
clients. If one types a text search query in a browser it then sends
proper UTF-8 characters and application after all needed processing
(escaping, checks, etc) passes it to database. But if one modifies URL
of the query adding some trash non-UTF-8 characters, database raises
an error: invalid byte sequence for encoding "UTF8".

What is the best practice to process such a broken strings before
passing them to PostgreSQL? Iconv from utf-8 to utf-8 dropping bad
characters?

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Regards,
 Ivan