Substring expression fails on single character input

When using the expression *'(\S.*\S)' *to remove surrounding whitespace
(including \t and \n) it returns NULL on single character surrounded by
whitespace.

The following shows the unexpected result:

SELECT x, '|'||substring(x, '(\S.*\S)')||'|'
FROM (VALUES (' a'), (' ab'), (' a b c '), (E' c\n'), (E' ab\n')) AS z (x);
x │ ?column?
──────────┼──────────
a │ <=== ERROR
ab │ |ab|
a b c │ |a b c|
c ↵│ <=== ERROR
│
ab ↵│ |ab|

Regards,
Per-Åke Ling