在 fish 前后使用负字符类，像这样:\(^\|[^[:alnum:]]\)fish\($\|[^[:alnum:]]\).这说:

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

1. 行首或任何非字母数字
2. 跟着鱼
3. 后跟行尾或任何非字母数字

这保证紧接在 fish 之前和之后的字符不是字母数字.

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

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