Postgresql – using regexp substitution part in postgres string functions

I hit this question trying to answer one in SO. When I pass \1 as first argument to lpad(string text, length int [, fill text]) function, it looks like it interprets \1 as regexp substitution part looking for string to pad chars, but then interprets same \1 as text (it is literally – '\1') calculating its length to substract from second argument: lpad('aaa',5,'b') calculates 5-length('\1'), not 5-length('aaa').

So the question is – are we meant to use SQL string functions with regexp substitution part meta syntax? What are the restrictions?
Eg: concat, format, repeat, substr understand it, while length, initcap, ascii interpret it as just text.

Now I see that length, initcap, ascii accept string as argument, while format, repeat accept text, but then ltrim(text... and lpad(text... seem to be mix – they "understand" regexp metasyntax to find position and then fail to "convert string to text" to count input length?..

Here an example:

t=# with a as (select 'a 2 b'::text s)
t-# select
t-#   regexp_replace(s,' (\d){1} ','')
t-# , regexp_replace(s,' (\d){1} ','\1') _regular
t-# , regexp_replace(s,' (\d){1} ',concat('.','\1','.')) concat_regular
t-# , regexp_replace(s,' (\d){1} ',length('.\1')::text) length_string
t-# , regexp_replace(s,' (\d){1} ',reverse('\1')) reverse_string
t-# , regexp_replace(s,' (\d){1} ',initcap('\1')) initcap_broken
t-# , initcap(regexp_replace(s,' (\d){1} ','\1')) initcap_orig
t-# , regexp_replace(s,' (\d){1} ',ltrim('\1','a')) ltrim_broken
t-# , regexp_replace(s,' (\d){1} ',lpad('\1',5,'.')) lpad_broken
t-# , regexp_replace(s,' (\d){1} ',ascii('\1')::text) ascii_broken
t-# , regexp_replace(s,' (\d){1} ',format('\1')::text) format_regular
t-# , regexp_replace(s,' (\d){1} ',repeat('\1',3)) repeat_regular
t-# from a
t-# ;
 regexp_replace | _regular | concat_regular | length_string | reverse_string | initcap_broken | initcap_orig | ltrim_broken | lpad_broken | ascii_broken | format_regular | repe
at_regular
----------------+----------+----------------+---------------+----------------+----------------+--------------+--------------+-------------+--------------+----------------+-----
-----------
 ab             | a2b      | a.2.b          | a3b           | a1\b           | a2b            | A2b          | a2b          | a...2b      | a92b         | a2b            | a222
b
(1 row)

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