You can use LLFF. A sample code (w/o any error handling)

lhIn = FOPEN(lcFileIn)
lhOut = FCREATE(lcFileOut)
DO WHILE NOT FEOF(lhIn )
  lcLine = FGETS(lhIn,8192)
  * Processing here
  = FPUTS(lhOut, lcLine)
ENDDO
FCLOSE(lhIn)
FCLOSE(lhOut)

>
>I have a huge text file, could exceed 130 MB and I would like to search for specific string in this file and replae it with other string.
>EX. find "AD00" and replace it with "BR77".
>