Comment 3 for bug 1484296

It turns out the site that experienced the error was running PHP 5.5.12, while my machine is running PHP 5.5.9.

In 5.5.10 they updated the PCRE version from 8.31 to 8.34 ( http://php.net/ChangeLog-5.php#5.5.10 ). And in the release notes for PCRE 8.34 ( http://www.pcre.org/original/changelog.txt ), there's this: "23. Perl now gives an error for missing closing braces after \x{... instead of treating the string as literal. PCRE now does the same."

So that's the reason why the code did not cause any problems on my machine, but did cause problems on the other site.

It also probably means that the regex was not fully functional even if it wasn't giving out any errors. Probably the reason it worked for me was that my test data was using illegal characters in the ASCII range, which are at the start of the regex, before the part with the syntax error.