How do i use pattern matching to exclude certain characters in sed?

maltiger

Sample file used:

cat linefile              
This is line one
1000
This is line two
2000
This is line three

We want to keep characters This, that's T or t, h, i, and s, we put a ^ symbol in front in the square brackets.

$ sed 's/[^This]/ /g' linefile

Result: Including keeping blanks and spaces, goodness know why anyone would want a file like that, but this is just to show the usage.
This is i

This is i

This is i h