There are additional functions that you can use to filter strings which you can find in the SPARQL spec and also in our docs.
P.S.: Note that not for all of those functions there is an equivalent rewriting in (all dialects of) SQL. In those cases, the filters have to be evaluated at the Stardog server after the results are obtained from the Virtual Graph which can come with some performance penalties.
At the moment there is no overview on which functions can be evaluated by which SQL dialect. An easy way check it is hitting "Show Plan" in Studio and inspect plan to see whether the filter was pushed into the VirtualGraphSql node.