fast find strig in txt file with approx 700.000!!!!!!

I have a txt file in c:\mydir\....txt
i need to find a string in mid position of line 2, 6 the word AAAAAA.

when the string is found , loop all lines of txt from the line founded with AAAAAA until in mid position 2, 6 is BBBBBB...

i hope i'm clear:)

note:
approx lines in ttxt 700.000