Xjyuk

For some reason, using a variable as index of a list-item does not work for retrieving that item. But using a number instead of a variable works. Why?

documentclassstandalone
usepackageexpl3
ExplSyntaxOn
begindocument

 clist_new:NmyList
 clist_set:NnmyList1,south

 int_new:NmyCounter
 int_set:NnmyCounter1
 int_do_until:nNnnmyCounter=clist_count:NmyList+1
 regex_set:NnmyExp[0-9]+
 % following line doesn't work
 %regex_match:NnTmyExpclist_item:NnmyListint_use:NmyCounter
 % but as soon as we change int_use:NmyCounter to 1, it works
 regex_match:NnTmyExpclist_item:NnmyList1
 (is a number) % output
 
 int_incr:NmyCounter
 

enddocument

You are using a regex against the literal text clist_item:NnmyListint_use:NmyCounter, not the result of executing it. In the case where you use an explicit 1, that works, but not when you have no 1 present.

Fixing that by expansion, and also using standard variable names plus a step loop (seems easier here), I'd have

documentclassstandalone
usepackageexpl3

begindocument
ExplSyntaxOn
 clist_set:Nn l_tmpa_clist 1 , south 
 regex_set:Nn l_tmpa_regex [0-9]+ 
 int_step_inline:nn clist_count:N l_tmpa_clist 
 
 exp_args:NNf regex_match:NnT l_tmpa_regex
 clist_item:Nn l_tmpa_clist #1 
 (is ~ a ~ number ) 
 
ExplSyntaxOff

enddocument

The argument type n in regex_match:NnT states that the argument is passed “literally”, with no processing, but you need the item, not the instructions needed to retrieve it. You could define a variant

prg_generate_conditional_variant:Nnn regex_match:nn ne T,F,TF 

and use regex_match:NeT, but it's actually much easier with clist_map_inline:Nn that will access the clist's items one at a time:

documentclassarticle
usepackageexpl3

begindocument

ExplSyntaxOn

clist_new:N l_bp_mylist_clist
clist_set:Nn l_bp_mylist_clist 1,south

clist_map_inline:Nn l_bp_mylist_clist
 
 regex_match:nnTF [0-9]+ #1 
 #1~is~a~number par 
 #1~is~not~a~number par 
 

ExplSyntaxOff

enddocument

If you need to use the item number,

documentclassarticle
usepackageexpl3

begindocument

ExplSyntaxOn

clist_new:N l_bp_mylist_clist
clist_set:Nn l_bp_mylist_clist 1,south

prg_generate_conditional_variant:Nnn regex_match:nn ne T,F,TF 

int_step_inline:nn clist_count:N l_bp_mylist_clist 
 
 regex_match:neTF [0-9]+ clist_item:Nn l_bp_mylist_clist #1 
 Item~#1~`clist_item:Nn l_bp_mylist_clist #1 '~is~a~number par 
 Item~#1~`clist_item:Nn l_bp_mylist_clist #1 '~is~not~a~number par 
 

ExplSyntaxOff

enddocument