preg_match() use is overkill in path_is_absolute()

[coffee2code]

path_is_absolute() utilizes preg_match() to check if the first character of a path string is either of two possibilities. This is way overkill for such a simple case. The alternative I've proposed via the attached patch is more performant and makes the intention of the code clearer (rather than obscured in a regular expression).

Like the patch, but our coding standards (or at least standard usage) are for array style access to characters i.e. $str[0]. See #13900.

Ah, missed that convention. Thanks! Revised patch, 17754.2.diff, attached.

Square bracket syntax, rather than curly

In [18551]:

Replace regex in path_is_absolute() with simpler equality checks. props coffee2code. fixes #17754