mjhjmtu

I've got a Dell Latitude and eventually use xedit under cygwin-X (and via ssh -X also on Linux machines). I need the CTRL-_ key to undo edits. That key combination is recognized by e.g. xemacs (for a different purpose).
As far as I can see the keystroke in X applications is received as ASCII 31 ("Unit Separator").
What code does xedit expect, and if it isn't ASCII 31, how can I map it to the expected one?

X applications interpret events, not keystrokes. If you ran xev, you might see something like this (underscore is a shifted key on my keyboard):

KeyPress event, serial 34, synthetic NO, window 0xe00001,
 root 0x111, subw 0xe00002, time 1812782646, (61,35), root:(81,78),
 state 0x0, keycode 67 (keysym 0xffe3, Control_L), same_screen YES,
 XLookupString gives 0 bytes: 
 XmbLookupString gives 0 bytes: 
 XFilterEvent returns: False

KeyPress event, serial 34, synthetic NO, window 0xe00001,
 root 0x111, subw 0xe00002, time 1812783382, (61,35), root:(81,78),
 state 0x4, keycode 68 (keysym 0xffe2, Shift_R), same_screen YES,
 XLookupString gives 0 bytes: 
 XmbLookupString gives 0 bytes: 
 XFilterEvent returns: False

KeyPress event, serial 34, synthetic NO, window 0xe00001,
 root 0x111, subw 0xe00002, time 1812784150, (61,35), root:(81,78),
 state 0x5, keycode 35 (keysym 0x5f, underscore), same_screen YES,
 XLookupString gives 1 bytes: (1f) ""
 XmbLookupString gives 1 bytes: (1f) ""
 XFilterEvent returns: False

KeyRelease event, serial 34, synthetic NO, window 0xe00001,
 root 0x111, subw 0xe00002, time 1812784374, (61,35), root:(81,78),
 state 0x5, keycode 35 (keysym 0x5f, underscore), same_screen YES,
 XLookupString gives 1 bytes: (1f) ""
 XFilterEvent returns: False

KeyRelease event, serial 34, synthetic NO, window 0xe00001,
 root 0x111, subw 0xe00002, time 1812785271, (61,35), root:(81,78),
 state 0x5, keycode 68 (keysym 0xffe2, Shift_R), same_screen YES,
 XLookupString gives 0 bytes: 
 XFilterEvent returns: False

KeyRelease event, serial 34, synthetic NO, window 0xe00001,
 root 0x111, subw 0xe00002, time 1812785335, (61,35), root:(81,78),
 state 0x4, keycode 67 (keysym 0xffe3, Control_L), same_screen YES,
 XLookupString gives 0 bytes: 
 XFilterEvent returns: False

xedit uses the X Toolkit translations resource to convert those events into an action, e.g., using the default resource-setting compiled into TextTr.c:

":c<Key>_:" "undo()n"

So (unless you're overriding the default resources), it's simply the control modifier plus the underscore (however your keyboard produces it).

xedit would not recognize the analogous ASCII ^_.