An additional \ causes Yara performance warnings

[swami-dtex]

yara.compile of
/[a-z]{1,63}[a-z]{2}[0-9]{3}-[0-9]{1,5}/
works but
/[a-z]{1,63}\[a-z]{2}[0-9]{3}-[0-9]{1,5}/ with an additional
throws a performance warning: 'may slow down scanning'.

Please help understand why this is the case. Is it possible that this is a bug with Yara?