Many wikis use an inputbox or TOC to produce links like https://en.wiktionary.org/wiki/Category:Chinese_reduplications?pagefrom=%E7%87%A6%E7%87%A6 , is this what you mean?

In T179269#3718862, @Nemo_bis wrote:

inputbox or TOC to produce links

Could you elaborate a bit please? I wuold add a searchbox just for a category's pages, or alternatively restrict the general searchbox to search just in the "current category"

The WMDE team is working on a new Advanced Search feature which will add in the ability to search for categories along with other fields: https://meta.wikimedia.org/wiki/WMDE_Technical_Wishes/AdvancedSearch. It's planned to go live in 2018, but I don't have exact dates, however, this might be what you're looking for.

In T179269#3721157, @debt wrote:

this might be what you're looking for.

Thanx so much for the info. It looks great. Do you know what kind of regular expressions (regex) it will allow?

In T179269#3721196, @Backinstadiums wrote:

In T179269#3721157, @debt wrote:

this might be what you're looking for.

Thanx so much for the info. It looks great. Do you know what kind of regular expressions (regex) it will allow?

@Backinstadiums -- @Bmueller or @gabriel-wmde would be able to answer your question. :)

In T179269#3721262, @debt wrote:

@Backinstadiums -- @Bmueller or @gabriel-wmde would be able to answer your question. :)

How can I ask them?

By writing a well-formed question here, now that they are CC'ed. :)

@Backinstadiums we are building an advancedSearch form and plan to include category search as well. In the category form itself you will most likely not have the option to use regex, but you will still be able to use the general search field, and could use our form to restrict the search to certain categories only. However, we are still in development, and category search will take a few more months to come.
(And I read your comment on the talk page first, so apologies for two slightly different answers :) )