manipulate list of words ( --> strings) - keep subset of strings with similar pattern

[Boris Molls]

I have a local with a list of string and want to keep a subset of the string that have the same pattern. I can't work out how to do it in a simple way with the the build in string and macro list functions.

Problem
I have the following local (list).

local my_large_string_list "a1 a_2 a_x zx zc zv a"

I want to create the list
local my_small_a_list "a1 a_2 a_x a"

Working with variables I would write "keep a*". How do I do something similar to the functionality * (star)? Alternative, how do I create a 'not' list that I subtract from the full list.
(Note: my list is too large to do the above manually. This is a simplification of my actual task.)

Additional explanation
I tried to do this in the framework of
local my_small_a_list : { command } local my_large_string_list { command }.
For example to remove zx I would use: local no_zx_list : subinstr local my_large_string_list "zx" "", all
Essentially, I want to do the opposite. Not to remove but to keep. And I want to keep with a short cut for keeping everything with a particular substring.

Can someone help?

Thanks,
Boris

[Andrew Musau]

local my_large_string_list "a1 a_2 a_x zx zc zv a"
local remove= trim(itrim(ustrregexra(" "+ "`my_large_string_list'" + " ", "[^ ]*a[^ ]*", " ")))
local wanted: list my_large_string_list - remove
di "`remove'"
di "`wanted'"

Res.:

 
. di "`remove'"
zx zc zv

. 
. di "`wanted'"
a1 a_2 a_x a

[Boris Molls]

Thanks, Andrew!

Can you point me to the documentation or page where I can read up on the syntax in the expression "[^ ]*a[^ ]*"

I can't find it.

[Andrew Musau]

Stata's -ustrregex*()- functions use the ICU regular expression engine (see Hua Peng's post #16 at https://www.statalist.org/forums/for...ressions/page2). The official documentation is at https://unicode-org.github.io/icu/us...gs/regexp.html. If you find the proposed solution unintuitive, note that is possible to solve the problem without using regular expressions.

local wanted
local my_large_string_list "a1 a_2 a_x zx zc zv a"
foreach w of local my_large_string_list{
    if strpos("`w'", "a")>0{
        local wanted "`wanted' `w'"
    }
}
display "`wanted'"

Res.:

. display "`wanted'"
 a1 a_2 a_x a