Done with: sed -in 's/\(ifndef\|define\) _\([0-9A-Z_]*\)$/\1 I3_\2/' include/**/*.h fixes #804
e.g. pressing Mod1+x when having the following in your configfile: bindsym Mod1+x some invalid command will lead to an i3-nagbar instance popping up, offering you to view the error log (which will contain parser errors from this commit on).