pcrepattern specification - Perl Compatible Regular Expressions
https://pcre.org/original/doc/html/pcrepattern.html
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be changed in the same way as the Perl-compatible options by using the characters J, U and X respectively. When one of these option changes occurs at top level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows.
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