RegEx failing since PHP 7.4, working in 7.3

Any ideas why this preg_match works up to PHP7.2 but fails with 7.3+ ?

$word = 'umweltfreundilch'; //real life example :/
preg_match('/^(?U)(.*(?:[aeiouyäöü])(?:[^aeiouyäöü]))(?X)(.*)$/u', $word, $matches);
var_dump($matches);

Warning: preg_match(): Compilation failed: unrecognized character after (? or (?-

PHP 7.2 and below output:

array(3) {
  [0]=>
  string(16) "umweltfreundilch"
  [1]=>
  string(2) "um"
  [2]=>
  string(14) "weltfreundilch"
}

RegEx seems to be ok, doesn't it?
https://regex101.com/r/LGdhaM/1

Solution

In PHP 7.3 and later, the Perl-Compatible Regular Expressions (PCRE) extension was upgraded to PCRE2.

The PCRE2 syntax documentation does not list (?X) as an available inline modifier option. Here are the supported options:

  (?i)            caseless
  (?J)            allow duplicate named groups
  (?m)            multiline
  (?n)            no auto capture
  (?s)            single line (dotall)
  (?U)            default ungreedy (lazy)
  (?x)            extended: ignore white space except in classes
  (?xx)           as (?x) but also ignore space and tab in classes
  (?-...)         unset option(s)
  (?^)            unset imnsx options

However, you may actually use X flag after the trailing delimiter:

preg_match('/^(?U)(.*[aeiouyäöü][^aeiouyäöü])(.*)$/Xu', $word, $matches)

See PHP 7.4 demo.

To cancel (?U) effect, you may use either of the two options: a (?-U) inline modifier, like in

preg_match('/^(?U)(.*[aeiouyäöü][^aeiouyäöü])(?-U)(.*)$/u', $word, $matches);
//                                           ^^^^^

Or, enclose the affected patterns into a (?U:...) modifier group:

preg_match('/^(?U:(.*[aeiouyäöü][^aeiouyäöü]))(.*)$/u', $word, $matches);
//            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

See more about changes to regex handling in PHP 7.3+ in preg_match(): Compilation failed: invalid range in character class at offset.