regexTransformer returns no results if there is no match

[alxsss]

Hello,

I try to construct wikipedia page url from page title using regexTransformer
with


<field column="title_underscore"  regex="\s+" replaceWith="_"  
sourceColName="title" />

This does not work  for titles that have no space, so title_underscore for them 
is empty.

Any ideas what is wrong here?

This is with solr-4.8.1

Thanks. Alex.