how to find a string in a textfile?

I need to insert a specified string after an other specified string.

const STR1 = "blabla"
const STR2 = "something"
const FILENAME = "something.txt"

how to find STR1 in the FILENAME textfile?
how to insert STR2 after STR1?

Like this
filename = Replace(filename,STR1,STR1 & " " & STR2)