Eww what the hell. I don't use ruby, and if I did I might very well - as a PCRE half-expert - fall into this trap based purely on the assumption that ruby was using PCRE. I just looked at their Regexp class, and it matches PCRE in most regards. The fact that /m makes . match newline \n is horrible - every PCRE-based implementation uses /s for that, whereas /m only affects ^ and $.
It still falls on the developer to understand the exact flavor of regex available in their language. And yet ruby is doing a disservice to anybody coming to their language with existing PCRE knowledge by having syntax that is almost an exact match to PCRE used in many languages... only to find out someday that it's not. Harsh.
(and incidentally, that would make changing it easier, you can just request that users specify a flag all the time and deprecate the one without).