If the function is active, only entries that match a filter are processed. The filter is defined by the content of the topmost text box.

See Filter Format for a description of the filter syntax.

It is recommended to verify a filter before starting a lengthy conversion. If it is faulty, you may only see the error at the end of a long wait.Click the Verify button, and a possible problem is described, and the faulty part is highlighted. This only checks the syntax; content related errors like using non-existing field names can only be detected when conversion has started.

If you use Regular Expressions, the comparison is more powerful, but also more error prone. For example, this is fine in normal filters: comment="avoid open brackets [", but throws an error in regex filters. The assumption is, if you chose this, you know what you do :-) 

Note: when comparing to the special keywords *none* and *any*, they are not treated as regular expressions.

There are some more examples in the Filter Format section.

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