You probably have to do this in the client code after/during reading of the clob.
There is [TEIID-1248] Clob usability - JBoss Issue Tracker to cover adding built-in functionality. I think the only hesitancy about working it was ensuring that all pushdown forms of the functions worked as expected. Otherwise we have to add more capabilities or different function names e.g. locate_clob. You should vote for the issue so that it gets rescheduled for a release. As a workaround to keep things on the server-side, you can add user defined functions to cover the needed functionality.