Changing < to " but leave << alone in Vim

Question

I have a .cpp file in which I would like to change all less-than symbols to double quotes but maintain any instances of double-greater-than symbols, i.e. change < to " while leaving all occurrences of << alone.

I am very new to vim and as such am still trying to figure out the best way to replace characters in situations like this.

Answer 1

Just to make an answer out of my comment above: you could substitute all ocurences of < without a successor <:

%s/\(^\|[^<]\)<\($\|[^<]\)/\1"\2/g

Or, using the very-magic flag, as suggested by @d-ben-knoble:

%s/\v(^|[^<])\<($|[^<])/\1"\2/g

That means:

%s substitute in all lines

/\v starts search pattern and use very-magic flag, so we don't need to escape some characters

(^|[^<]) searches for a start of line (^) or one characters that is different from <

\< then searches for a < (the escape character here is needed)

($|[^<]) and finally searches for an end-of-line or a character that is different from <

/\1"\2/g and replaces all the findings with the first character, then " and the second character.

Answer 2

Use lookarounds

I think the simplest :substitute command might be to use lookarounds:

:%s/<\@<!<<\@!/"/g

This replaces every < character that is neither preceded nor followed by another < character.

How it works

It uses a :substitute command (See :help :substitute) to replace strings that match the regular expression <\@<!<<\@! with " characters.

The regular expression is reasonably simple when broken down. It consists of three parts:

:%s/<\@<!<<\@!/"/g
         ^
         1

In the middle is a single < character. This is what we want to match.

:%s/<\@<!<<\@!/"/g
    ^^^^^
      2

Before that is: <\@<!. This is a second < character followed by Vim's atom for negative lookbehind: \@<!, which means that our central < will only be matched if we didn't match a < here.

:%s/<\@<!<<\@!/"/g
          ^^^^
            3

At the end we have: <\@!. This is a third <, this time followed by Vim's atom for negative lookahead: \@!:our < will only be matched if it is not followed by a <.

See :help \@! and :help \@<! for more details of Vim's negative lookaround atoms.

Answer 3

It's tricky to match single <, you have to consider behind and after character, which might also be start of line or end of line. An alternative is to match both of them, use \= to deal with <<

%s/\V<<\|</\=submatch(0)=='<'?'"':'<<'/g

• \V<<\|< match << or < if no << exists
• \=submatch(0)=='<'?'"':'<<' replace < with ", << with <<
  • \= substitute with an expression, check :h sub-replace-expression
  • submatch(0) the whole matched text in current :substitute command.
  • submatch(0)=='<'?'"':'<<' return " for <, << for <<