Describe the bug

The regular expression that the PHP parser uses the (?<name>pattern) syntax to create named subpatterns. This syntax was introduced in PHP 5.2.2.

Strictly speaking, however, the syntax was introduced in PCRE 7.0, which is the default version of PCRE built into PHP 5.2.2, but some Linux distros (Centos 5 is a known example) use an older PCRE. In the case of Centos 5, they use PCRE 6.6.

In core (as well as in other places in Gutenberg), we've avoided this problem by using the older (?P<name>pattern) syntax.

I'd like to get this fixed in WordPress 5.1.

Ref: https://core.trac.wordpress.org/ticket/45644