The REGEXP_REPLACE function may have these arguments:
REGEXP_INSTR(String to search, Pattern, [Position,
[Occurrence, [Return-option, [Parameters]]]])
Replace everything from one lowercase "t" to the next with some field
SQL>
SQL> SELECT REGEXP_REPLACE('This is a test','t.+t','XYZ') FROM dual;
REGEXP_REPLAC
-------------
This is a XYZ
SQL>