sed exception handling of escaped characters

Question

I want to delete anything that follows a % to the end of the line. Using

cat /tmp/foo.txt | sed 's/%.*$//'

works great with one exception: I want to ignore any escaped percent signs \%. So with the following file saved as /tmp/foo.txt

abcd %123
xyz \%xyz
xyz \%xyz %123

the output I want is

abcd 
xyz \%xyz
xyz \%xyz

What is the appropriate regular exception handling to do this?

Answer 1

You need to look at the character before the percent.

sed 's/\([^\\]\)%.*/\1/'

If the previous character is not a backslash, keep that char and remove the rest.

This answer assumes that the % does not appear at the beginning of the line. If it does, then we need to check for it

sed 's/\(^\|[^\\]\)%.*/\1/'