Question

The documentation says that \s is whitespace and \S is not whitespace. So far, nothing new to regex users.

But let’s check some return values:

SELECT SUBSTRING('abc a c' FROM 'a\\sc');
'a c'

SELECT SUBSTRING('abc a c' FROM 'a[\\s]c'); -- Note the character class
'a c'

SELECT SUBSTRING('abc a c' FROM 'a\\Sc');
'abc'

SELECT SUBSTRING('abc a c' FROM 'a[\\S]c'); -- Note the character class
ERROR:  invalid regular expression: invalid escape \ sequence

So it seems, \s can be used in a character class and \S cannot. Why?

Answer 1

From the manual:

Within bracket expressions, \d, \s, and \w lose their outer brackets,
and \D, \S, and \W are illegal.

In any case, the brackets seem redundant since \s and \S themselves are character classes.

The following syntax works for me as an alternative to a[\\S]c:

SELECT SUBSTRING('abc a c' FROM 'a[^[:space:]]c');