When typing in an item code (for matrix item) on the global search or into a line on the transactions, it shows the matrix code first and then the child code. client have very long codes and this means they cant see the child matrix item code easily. Is there anyway of disabling this or writing a custom code for this?

Try to mark the "ONLY SHOW LAST SUBITEM" at Set Preferences

thanks @Yadigar

As far as I know you there in no option for this at the general preferences. However, you can set it per the roles preferences.