Weirdly enough when I use a single line pattern it starts working:

<?php

$pattern = '~^(?:(?:(?P<function>function)\s*\(\s*(?:(?P<table1>.+)(?:\.))?(?P<column1>[^ ]+)\s*\))|(?:(?P<table2>.+)(?:\.))?(?P<column2>[^ ]+))(?:\s+as\s+(?P<alias>.+))?$~ix';

var_dump(PCRE_VERSION);
var_dump(preg_match($pattern, 'id'));
var_dump(preg_last_error());

-----

string(16) "10.32 2018-09-10"
int(1)
int(0)

Am I doing something wrong here in combination with the platform I am on somehow?

Works for my 7.3.3 on Windows. Does it work if you copy/paste the code from this bug report? Is there maybe a stray unprintable character in your file?

Nope. Same result. When I check a hex dump of my file I also do not see anything strange at first sight.

Found it. The problem is the line ending in the flags part: PHP will ignore \n but not \r, then complain about it as an invalid flag.
https://github.com/php/php-src/blob/PHP-7.3.3/ext/pcre/php_pcre.c#L696

When I tested, I had created the file with LF line endings.

What's more, the error message is confusing because it says this (with Xdebug) in cmd, where the \r causes a carriage return and so overwrites the rest of the message:

  string(16) "10.32 2018-09-10"

  ' in C:\Users\xxxxxx\t.php on line 17er '

  Call Stack:
      0.4049     397800   1. {main}() C:\Users\xxxxxx\t.php:0
      0.4051     397832   2. preg_match() C:\Users\xxxxxx\t.php:17

  C:\Users\xxxxxx\t.php:17:
  bool(false)
  C:\Users\xxxxxx\t.php:18:
  int(1)

The error message was actually

  Warning: preg_match(): Unknown modifier ''

with an \r between the apostrophes.

Awesome! Thanks for digging into it and confirming I am not going entirely crazy yet.