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What the devil language is that in?? I see a /D flag, and you’ve quoted it with single quotes yet also used slashes to delimit the pattern? It’s not Perl, and it can’t be PCRE. Is it therefore PHP? I believe those are the only three that allow recursion like (?1).
It's in PHP, which uses PCRE. The slashes are used only to delimit special characters like parentheses, square brackets, and of course slashes and single quotes. The /D flag, if you didn't know, is to prevent a newline being added to the end of the string, which would be allowed otherwise.
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/Dflag, and you’ve quoted it with single quotes yet also used slashes to delimit the pattern? It’s not Perl, and it can’t be PCRE. Is it therefore PHP? I believe those are the only three that allow recursion like(?1).