This regex includes the character class [^a-zA-Z0-9_\\x7f-\\xff] (with backslashes escaped for JSON). The \x7f-\xff range in this negated class is triggering an Oniguruma bug, resulting in a bug in the Hack grammar. Although that range appears like it excludes code points U+7F to U+FF, in fact it excludes U+7F to U+10FFFF. The reason is described in this comment.

This can be fixed by simply changing the \xff to \x{ff}, which changes Oniguruma from interpreting it as an invalid standalone encoded byte value (true for unenclosed \xHH for values above 7F) to a code point value (always true for the enclosed form \x{...}). Note that this handling is specific to Oniguruma, not other regex flavors.

Also note that this is the only place in the grammar that \\x7f-\\xff appears, but the correct version \\x{7f}-\\x{ff} appears 28 times.

To fix this, the \\xff should be replaced with \\x{ff}. It will then work correctly in Oniguruma. It is optional for the \\x7f to also be replaced with \\x{7f}.