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Global find/replace in metadata WITHOUT affecting other text in same field

We are using CS4 and CS6 (yes, old school... but they work!) We have a library of about 30,000 images, all of which have metadata in the Description field. We want to change one particular character string in that field to another specific character string throughout ALL of the images WITHOUT erasing/changing the rest of the data in the field. The character string is not in the same place in every image (i.e., it is not always the last line.) We know how to do an automated batch job to simply add/delete text from a given field, but we're not sure if that will work for a find/replace job. We would prefer to NOT have to use a Java app or anything like; preferably it's something we can do within Photoshop itself. Any help would be greatly appreciated. Thanks.