Question

I am using sed to find and replace items, e.g.:

sed -i 's/fish/bear/g' ./file.txt

I want to limit this to only change items which do not have a letter or number before or after, e.g.:

• The fish ate the worm. would change, because only spaces are before and after.
• The lionfish ate the worm. would not change, because there is a letter before fish.

How can I find and replace some items, but not if at least one letter or number appears immediately before or after?

Answer 1

Use a negative character class before and after fish, like so: \(^\|[^[:alnum:]]\)fish\($\|[^[:alnum:]]\). This says:

1. Start of line or anything that’s not alphanumeric
2. Followed by fish
3. Followed by end of line or anything that’s not alphanumeric

This guarantees that the characters immediately preceding and immediately following fish are not alphanumeric.

sed 's/\(^\|[^[:alnum:]]\)fish\($\|[^[:alnum:]]\)/\1bear\2/g'