create and populate a field when building an index

[Keith Beef]

I've been using the Lucene demo from
http://lucene.apache.org/java/2_1_0/demo.html

I have a set of documents
with filenames that give a good indication of content.

A filename of 12 digits (I think this is [0-9]{12} as a regular
expression) with the extension html is a troubleshooting guide, the number
being an error code. A filename with two or three letters, then a minus
(which would be [a-z]{2,3}- I think), then a known string means the
document is about a particular subject; I have a list of the known strings
matched to subjects.

What I would like to do, is have my indexer create a field named
"category", populated with either the string "troubleshooting" or with the
known string extracted from the filename.

Examples:
For a file named 0000000000111.html the indexer adds the field "category"
with the value "troubleshooting". For a file named xxx-cal-123.html the
indexer adds the field "category" with the value "cal". For a file named
xx-qv-(9).html the indexer adds the field "category" with the value "qv".

Is there a way to do that?

Beef.