pcre - Regex Substitution: only replace the first occurance from a "or" condition

Question

I need to replace d+ and a+ to x, I want to replace all the d+ to x, but for a+, I only want to replace the first occurance if there's any.

Is it possible to use only one regex to achieve this?

Input	Desired Output
123,456,789	x,x,x
123,456,a,789	x,x,x,x
aaa,aa,aaa	x,aa,aaa
aa,123,456,aaaa,789	x,x,x,aaaa,x
aaa,123,456,aa,789,aaaa	x,x,x,aa,x,aaaa

question from:https://stackoverflow.com/questions/65934161/regex-substitution-only-replace-the-first-occurance-from-a-or-condition

Answer 1

Sorry I forget that answer must be posted aside and post it in comments at first.

Now here it is:

Regex: d+|((?<!a|(?!(?1)).))a+

Replacement: x

https://regex101.com/r/v24o7B/1