I use sed to replace digits regex capture group.
How to use d with sed?
This works:

$ echo 'fxLayoutGap="10px"' | sed -E 's/fxLayoutGap="(.*)px/class="gap-1/'
class="gap-10"

This does not work. Why?

$ echo 'fxLayoutGap="10px"' | sed -E 's/fxLayoutGap="(d*)px/class="gap-1/'
fxLayoutGap="10px"

$ sed --version
sed (GNU sed) 4.7