After you find the REDEFINES, I think you should search for the prior ' 01 ' and search for the delimiter character that follows REDEFINES.
When I have done such searching, I have made a copy of the data, then blanked out quoted strings and comments. Also, names of objects pose a problem. You want avoid processing your search text that occurs out-of-context, for example:
01 MYREDEFINES PIC X(10)
Make a copy, process the copy, then map that to the original file to extract the lines needed. Yeah, it is non-trivial.