Vim: Expressions in replacement text

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 divs to hexadecimal after decrementing them by 1.

Basically, I had to convert

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

<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