Vim advanced search and replace

01-07-2012
Tagged vim, development, ruby

When I get a few days off from work, I do what every self-respecting geek would do: I dive into my text editor and try to learn some new stuff. Here’s a few tricks I learned recently concerning search and replace in Vim.

1. Indenting a multi-line search match

Say you have a Markdown document with code blocks marked up in Github-style ‘fences’:

Here is a code example:

```ruby
puts "Hello, world"
```

You want to convert these code blocks to standard Markdown code blocks – which have no fences, but instead are indented four spaces. This is desired end result:

Here is a code example:

    puts "Hello, world"

The example includes only a single code block with a single line of code, but let’s assume there’s several multi-line code blocks. How to go about this problem?

My first attempt was to use a regular expression search to match the entire code block, and then find some way to perform the indent command on its matches. The regular expression is not too hard, once you know the relevant Vim regular expression syntax:

/\v^```ruby\_.{-1,}```$/

Note the \v for very magic syntax, \_. to match any character (including newlines) and {-1,} to non-greedily match at least one of \_..

This search matches all the fenced code blocks alright, so we need a way to operate on every match. Here the global command (g) comes into play. Here’s my first attempt to indent every match:

:g/\v```ruby\_.{-1,}```$/>

The global command allows you to operate on every line matching a pattern, in this case using the > command to indent lines. Alas, only the first line of the match would indent, giving me:

Here is a code example:

    ```ruby
puts "Hello, world"
```

My misunderstanding was that a multi-line search would match multiple lines – instead, a search in Vim is found on a single line (which makes sense when you think about it).

After some Googling for search operations, I found the global command sets the current line in vim to the line of the search match. From there, you can start a new range up to the closing fence and operate on that range with the indent command. Here’s how:

:g/^```ruby/.,/```$/>

So, to recap: g/^```ruby/ searches for our opening fence and sets Vim’s current line to that line. The rest of the command is the operation on that line. .,/```$/ is a range. We might express ranges with line numbers (i.e. 10,20), but we can also use special symbols and searches. In this case, . is the current line (the one with the opening fence), and we end our range on the first match from our current line that matches our closing line. Finally, we can operate on a range using any ex command1, in this case > to indent. Success!

2. Solve your problems with external tools

It later occured to me the above problem might just as well have been solved using Ruby. Here’s an example solution:

:%!ruby -pe "gsub /^/, '    ' if /^```ruby/../```$/"

We use %! to filter the entire file through the ruby program. The -p flag to Ruby tells it to loop over STDIN and print every line back to STDOUT. The -e flag to Ruby evaluates a string of Ruby code. Since -p put Ruby in the special command-line mode, gsub and and regular expressions are implicitly sent to the current line. The above string of Ruby code is equivalent to the following program:

while $_ = STDIN.gets
  if $_ =~ /^```ruby/../```$/
    $_.gsub! /^/, '    '
  end
  print $_
end

Finally, the regular expression range syntax (also known as the flip-flop operator) is one of those awesome Ruby tricks few people know: looping over several lines, it becomes true when the left-hand regex matches, and stays true on subsequent tests until the right-hand side regex matches.

Is the Ruby alternative more readable than the Vim one? You decide for yourself – just know there’s more than one way to skin a cat.

3. Adding an incrementing counter to search matches

A while back I had an HTML document with sixteen headers I wanted to prefix with an incrementing number. I wanted to turn this:

<h3>Heading 1</h3>
<p>Lorem ipsum</p>
<h3>Heading 2</h3>
<p>Lorem ipsum</p>

…into this:

<h3>1. Heading 1</h3>
<p>Lorem ipsum</p>
<h3>2. Heading 2</h3>
<p>Lorem ipsum</p>

I accomplished it with Vim’s search-and-replace capabilities and a little vimscript, taking advantage of \=, which allows us to use an expression in the substitue string.

The first step to solving this problem is matching the right characters for replacement. In this case, we want to insert our counter right after the opening heading tag. We can match that exact position like so:

/\v\<h3\>\zs/

We have to escape the < and > because in Vim’s regular expression syntax they represent left en right word boundaries. \zs is Vim’s positive look-behind syntax, causing Vim to look for our opening heading tag, but only starting our match after it.

Using vimscript to increment a counter is not too hard, either. We could do it like so:

let i=1
" insert i in the document somehow
let i=i+1

My first try to combine these was to just use a regular substitue command, combining it with the vimscript lines for the counter:

:let i=1 | %s/\v\<h3\>\zs/\=i/ | let i=i+1

You can use | to combine multiple statements into a single command. \=i will use the current value of i as the the substituion string. But here’s the end result:

<h3>1Heading 1</h3>
<p>Lorem ipsum</p>
<h3>1Heading 2</h3>
<p>Lorem ipsum</p>

Vim has inserted the same value of i on every match. Here’s why: %s is a single operation on a range (%) that includes the entire document. If only we could make a single replacement for every line that matched our search… The global command to the rescue, again:

:let i=1 | g/\v\<h3\>\zs/s//\=i/ | let i=i+1

I replaced the % range with a global command using the same search pattern. To not repeat myself, I removed the search pattern from the substitution command, causing vim to re-use to last used pattern – which in this case is the pattern from the global command. Miraculously, it works:

<h3>1Heading 1</h3>
<p>Lorem ipsum</p>
<h3>2Heading 2</h3>
<p>Lorem ipsum</p>

We just need to add a little extra text to the substitution string:

:let i=1 | g/\v\<h3\>\zs/s//\=i.". "/ | let i=i+1

As you can see, we can concatenate the string ". " to our counter using the . operator, giving us the end result:

<h3>1. Heading 1</h3>
<p>Lorem ipsum</p>
<h3>2. Heading 2</h3>
<p>Lorem ipsum</p>

Another great success!

4. Editing complex commands

Admittedly, we have created quite a monster command using vimscript, a global command, a substition command, the special expression register and positive look-behinds. But Vim is a developer’s editor, and developers (should) know this stuff. It is not the most readable of code, but it is not horrible either.

The only truly awkward aspect of typing complex commands like these is entering them on Vim’s command line. There’s a remedy for that, though: the command line window.

The command line window is a new window (a split screen) in the current tab, that contains a buffer with your entire command history, one command per line. You can move around and edit commands just like in any buffer, making it easy to make changes. But when you press Enter in normal mode, it will execute the current line as if you had typed it on Vim’s command line, against the previously active window.

The command line window is a great help in iteratively building up complex commands. It allows you to quickly revisit previously issued commands, make some changes, and execute it again. When done, you can simply close it again using :q. But Vim wouldn’t be Vim if it hadn’t some awkardness, and this time it’s the q: shortcut to open it. It just way too similar to :q, and I open the command-line window by accident all the time.

  1. For a full list of available ex commands, search the Vim help system using: :help holy-grail.