Description:
------------
The documentation at https://www.php.net/manual/en/regexp.reference.delimiters.php says that:

When using the PCRE functions, it is required that the pattern is enclosed by delimiters. A delimiter can be any non-alphanumeric, non-backslash, non-whitespace character.

The validation for delimiters works in *most* cases and gives a warning such as:

Warning: preg_split(): Delimiter must not be alphanumeric or backslash in php shell code on line 1

However, it fails to detect the invalid delimiters in the string " *, *".

Instead, it runs without error/warning and gives output based on a substring(?) of the supplied regex.

According to https://3v4l.org/DdBp1 it affects all versions of PHP from 4.3 to 8.0

Test script:
---------------
# Valid delimiters - PASS
php > var_export(preg_split('/ *, */', 'a , b'));
array (
  0 => 'a',
  1 => 'b',
)

# Invalid delimiters - PASS - gives warning as expected
php > var_dump(preg_split('X *, *Y', 'a , b'));

Warning: preg_split(): Delimiter must not be alphanumeric or backslash in php shell code on line 1
bool(false)

# Invalid delimiters - FAIL - should warn about either mismatched or invalid delimiters.
# Instead, it gives no warning and unexpected output.
php > var_export(preg_split(' *, *', 'a , b'));
array (
  0 => 'a ',
  1 => 'b',
)


Expected result:
----------------
I expect the invalid delimiters to trigger a warning.

Actual result:
--------------
The invalid delimiters are silently ignored.
The output does not correspond to the supplied regex.