Oracle: How to create secondary index based on every word in a CLOB
You probably want to use a contextual index. Then when you search in the clob column you would do something like 

myclob contains ( 'aword') &gt; 0


This would then be a substitute for regexp. I have not used this
feature in a few years, but it does take some work to create and 
maintain the indexes. You should look for specific examples that 
people created and used the index.

Indexing with Oracle Text