“Gremlin” is my name for an invisible character other than a plain whitespace, a linefeed or a horizontal tab. Gremlins can cause errors in data processing and can also make it harder to detect duplicate records in a data table. The newest version of a gremlin detector script (for UTF-8-encoded plain text files) is demonstrated in this blog post, with notes on the sometimes difficult NUL byte.

Source: LXer – Gremlin detection bigly improved and a NUL problem avoided