At our last Ruby user group meeting it was asked, what the differences between CRuby and MRuby are. @junjis0203 wrote a MRuby FAQ. Unfortunately, it’s in Japanese. I’ve tried to translate it using http://www.excite.co.jp/world/english/ & Google translate and added my own findings.
Not all classes and method from CRuby are available in MRuby
MRuby does not guarantee to implement the features which are defined in ISO / IEC 30170
Mruby allows you to switch features on or off depending on your needs and available space. These features are defined in “include/mrbconf.h”. One of the features is regular expression support, which is switched off by default.