Question

With sed, I can replace one item with another. E.g.:

sed -i "s/a/b/g" file

However, there are times when I do not want this replacement to happen. How can I ensure that the replacement only happens when:

• The character immediately before is not a letter, number, or {.
• The character immediately after is not a letters or number, or }.

Answer 1

sed -r -e "s/([^0-9a-zA-Z\{])a([^0-9a-zA-Z\}])/\1b\2/g"

([^0-9a-zA-Z\{]) – is a first matched group of symbols (length of 1 in current case). Not a number (0-9), not a letter (a-zA-Z) and not a { (\{).

([^0-9a-zA-Z\}]) – is a second matched group, just like first.

If we matched those sequences, we substitute it with first group (\1), b and second group (\2).