public String getTargetField() {
        return targetField;
    }
This filter runs a regex against the first value in the "sourceField".
If the pattern matches, it extracts the first group of the first match and
set's the "targetField"'s value to that first group.
<p/>
If there is a match, this will overwrite whatever value is in the
"targetField".
<p/>
If there is not a match, this filter will be a no-op.
<p/>
If there are multiple matches, this filter will capture only the first.
Open a ticket if you need different behavior.
<p/>
If the source field has multiple values, this will run the regex
against only the first value.
<p/>
If the source field does not exist, this filter will be a no-op.
<p/>
If the target field is the same value as the source field, this filter
will overwrite the value in that field. Again, if there are multiple
values in that field, those will all be overwritten.
tika-core/src/main/java/org/apache/tika/metadata/filter/CaptureGroupMetadataFilter.java