The font configuration file specification prescribes that font names used in filename properties have space characters replaced by underscore characters. Presumable this was introduced in ancient times because space characters can't be used directly in property keys (but there is an escape syntax for them).

In the current implementation of FontConfiguration, the mapping is actually reversed, and the class has specific knowledge of a font name where the mapping should not be reversed because the font name actually includes an underscore character. This is rather fragile and would break if a new font showed up that also included an underscore in its name.

To remove this risk, the mapping from space to underscore (and back) should be removed from spec and implementation. The font configuration files would then have to use "\ " for " ".