restrict shell command to only part of a line

Question

In a file I'd like to replace some occurrences of triplets of 0 with F, 1 with E, ..., F with 0. Only some, so I can't just pipe the whole file through a bash tr filter; I need a text editor to examine each occurrence.

This is to invert the grayscale colors in some 1990's HTML from light-on-dark to dark-on-light, #000 to/from #FFF, #222 to/from #DDD,... .

What almost works: cursor to the first digit, ! tr 0123456789ABCDEF FEDCBA9876543210. After that I can just hit dot to repeat. But that also reverses any other digits on the line. For example, it changes

border: 2px solid #555;

to

border: Dpx solid #AAA;

• Can I restrict the command to affect just the 555?
• Or can I write a macro purely within vim that reverses this range of values, just like tr?
• Or can a command other than tr, applied to the whole line, limit its effect to /#[0-9]{3}/? Maybe some sed magic?

(If the last of these three turns out to be the solution, then this question may be off topic here.)

Answer 1

use :h :s\=

s/\v\#[0-9A-F]{3}[0-9A-F]@!/\=tr(submatch(0), '0123456789ABCDEF','FEDCBA9876543210')/g

• \#[0-9A-F]{3} match #000, #111, ... #FFF
• [0-9A-F]@! look forward to make sure #0000 doesn't match. check :h /\@!
• \= replace with an expression
• tr(submatch(0), '0123456789ABCDEF','FEDCBA9876543210') transform matching text

Since you want to do it manually, a command or map would help:

command InvertGrayScale s/\v\#[0-9A-F]{3}[0-9A-F]@!/\=tr(submatch(0), '0123456789ABCDEF','FEDCBA9876543210')/g

nnoremap <leader>ig :InvertGrayScale<cr>