+++
title = "Vim: Expressions in replacement text"
date = "2020-06-14"
author = "Ceda EI"
tags = ["vim", "neovim", "replacements"]
keywords = ["vim", "neovim", "replacements"]
description = "Modifying the selection in a :s[ubstitute]// command"
showFullContent = false
+++

Often times I find myself needing to modify the selection in a substitute command in a non-trivial way. From a recent example, I needed to convert the numbers in the `id` of the `div`s to hexadecimal after decrementing them by 1.

Basically, I had to convert

```html
<div id="someprefix1">
	 ...
</div>
<div id="someprefix2">
	...
</div>
<div id="someprefix3">
	...
</div>
<div id="someprefix4">
	...
</div>
...
<div id="someprefix80">
	...
</div>
```

to

```html
<div id="someprefix0">
	 ...
</div>
<div id="someprefix1">
	...
</div>
<div id="someprefix2">
	...
</div>
<div id="someprefix3">
	...
</div>
...
<div id="someprefix4F">
	...
</div>
```

Doing this manually is tiring, I could have used macros but I instead chose an even simpler approach.

Vim allows to use expressions in the replacement part of the substitute command if it starts with `\=`. So, the replacement can be simply done using `\=` and `printf`. What I did was simply use the following command after selecting the text in a visual selection.

`:'<,'>s/\v(someprefix)(\d+)/\=printf("%s%X", submatch(1), submatch(2) - 1)`

The breakdown is as follows:

- `:'<,'>`: Sets the range to visual selection.
- `s/`: Starts the substitution.
- `\v`: Enables "very magic mode". This allows for writing simpler regular expressions without having to escape special characters.
- `(someprefix)(\d+)`: Captures the string `someprefix` in group 1 and the number that follows in group 2.
- `/`: Ends the search and starts replacement.
- `\=`: Tells Vim to treat the following text as an expression.
- `printf(`: `printf` is a function that takes a format string and values for the format parameters.
- `"%s%X"`: The format string. `%s` means to print the parameter as a string. `%X` means to convert the parameter to a hexadecimal number in capital letters (`%x` for small).
- `submatch(1)`: It retrieves the first matched group.
- `submatch(2) - 1`: It retrieves the second matched group and subtracts one from it.

### References

- `:help sub-replace-expression`
- `:help submatch`
- `:help printf`
- `:help /magic`
- `:help :s`