regex - Ignore comments (#) using sed, but keep the lines untouched

Question

I have a config file that I want to basically edit the uncommented lines, but not the commented lines. I'm using sed.

For example, I have a file called file.txt:

test
# test
# test
test

I want to replace "test" with "TEST" but do NOT touch the commented lines. Final output should show:

TEST
# test
# test
TEST

Answer 1

sed '/^#/!s/test/TEST/g' /path/to/infile

Output

$ sed '/^#/!s/test/TEST/g' infile
TEST
# test
# test
TEST

*Note: If your only requirement for a comment is that the very first non-whitespace character is a #, then you can use:

sed '/^[[:space:]]*#/!s/test/TEST/g' /path/to/infile

Output

$ sed '/^[[:space:]]*#/!s/test/TEST/g' infile
TEST
# test
 # test
TEST