CLOB Replacement - RTF format data

Hi Folks

We have a requirement to be able to take a .rtf file (which we are storing in the database as a BLOB) and do a set of replacements (e.g. [last_name] will become smith) based on the data in some rows in Oracle. We thought this would be really simple - put the content into a CLOB, use a replace function (home-grown function) to replace the place-holders with the data and then move it back into the BLOB.

Except this doesn't seem to work consistently.

Before I go into any more details, wondering if any of you have done something similar and have it working reliably? Somehow it appears that we are sometimes "corrupting" the RTF so that the file, although complete, will not render correctly anymore. Is there anything "funny" about .rtf format that we should be paying particular attention to? It seems stange as we have no issues replacing text in other CLOBs.

This is on 9iR2

Thanks for any help

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jeremy