Regex & Sed/Perl: Match word that ISN'T preceded by another word

Question

I'd like to use sed or perl to replace all occurrences of a word that doesn't have a certain word in front of it.

For example, I have a text file that contains a plot of a movie and I want to replace all occurrences of a character's last name with their first name, but only if their first name doesn't come immediately before their last name.

Sample text might look like this:

John Smith and Jane Johnson talk about Smith's car.

I want it to look like this:

John Smith and Jane Johnson talk about John's car.

If I just do sed 's/Smith/John/' file, then I would have:

John John and Jane Johnson talk about John's car.

The first name that comes before the last name will always be the same. I don't have to deal with John Smith and Frank Smith. I just need a way to match Smith that doesn't have John preceding it.

Answer 1

Would be easy with any language where the regular expressions are capable to lookbehind. Of course, Perl is the first on list:

perl -pe 's/(?<!John\W)Smith/John/g' <<< "John Smith and Jane Johnson talk about Smith's car."

The weak point is having more than one non-word character between “John” and “Smith”. Unfortunately a quantifier like + for \W would raise “Variable length lookbehind not implemented” error.

Answer 2

EDIT .. re your comment.. Here is a new script which doesn't concern itself about (eg.) William Smith. It temporarily obfuscates patterns which it keeps as Smith (unchanged).

sed -r 's/\<(John) (Smith)\>/\1\x01x\2/g; 
        s/\<Smith\>/John/g;  s/\x01x/ /g'

If you are concerned about Mr. Mr Mrs... then this works.

sed -r 's/\<(John|((M(r|rs|s))\.?)) (Smith)\>/\1\x01x\5/g
        s/\<Smith\>/John/g; s/\x01x/ /g'

You can cater for William by adding his name to the or list, eg.
sed -r 's/\<(William|John|...

---

This is the orginal script

sed -r 's/(^|[[:punct:]] |\<[a-z]+ )(Smith\>)/\1John/'

Answer 3

 sed -r 's/([^John] )Smith/\1John/g;s/([^Jane] )Johnson/\1Jane/g'

The () will capture the non-Firstname before a LastName, so they are backref'd in the replacement.

Edit

@manatwork,gilles

You're right. How about

sed -r 's/(John Smith)/temp1/g;s/Smith/John/g;s/temp1/John Smith/g'

This seems to do the trick.