Finding and replacing part of a file
===

I have a text in a file file.txt like this

    xxxxxxxxxxxxxxx
    xxxxxxxxxxxxxxx
    a    b   c // delimited by tab
    xxxxxxxxxxxxxxx
    xxxxxxxxxxxxxxx

I know using `sed` I can find and replace text in a file. If a line starts with a   b(seperated by a tab) I need to replace it with d   e   f. So the above file will be 

    xxxxxxxxxxxxxxx
    xxxxxxxxxxxxxxx
    d    e   f // delimited by tab
    xxxxxxxxxxxxxxx
    xxxxxxxxxxxxxxx

I can do this to find and replace, I want only those instances where the line starts with a   b and replace the whole line.

    sed -i 's/a\/t\/b\/t\/c/REPLACED TEXT/g' file.TXT