Uppercase input turns on periodically

You can define your own table for the Compose key: create a file called .XCompose in your home directory. You'll need to define the whole table (you can't just add your definitions to the system default, you have to copy the system default into your file if you want it).

You'll find the system default table in /usr/share/X11/locale/en_US.UTF-8/Compose on Debian and Ubuntu and in a similar location on other unices. The format should be reasonably straightforward; a typical definition looks like these:

<dead_acute> <a> : "á" aacute         # a dead key: press Dead_acute then A to insert "á"
<Multi_key> <acute> <a> : "á" aacute  # Press Compose, ', A to insert "á"
<Multi_key> <g> <a> : "α" U03B1       # 03B1 is the hexadecimal code of "α"
<Multi_key> <M> <A> : "∀" U2200      # 2200 is the hexadecimal code of "∀"
<Multi_key> <minus> <greater> : "→" U2192
<Multi_key> <bar> <minus> <greater> : "↦" U21A6

ASCII characters and a few others have symbolic names; you'll find these names in /usr/include/X11/keysymdef.h. For example, - is minus because keysymdef.h contains the line #define XK_minus 0x002d and 2d is the hexadecimal code of -.

You can have sequences of more than two characters, like |-> in the example above. Note that if you define a compose sequence for |->, you can't have another for |-.

If you prefer AltGr+key₁ʹ, key₂ to Compose, key₁, key₂ then you can bind AltGr+key₁ʹ to a dead key with xmodmap. It's not necessarily a blessing, and note that you are limited to the dead key names listed in /usr/include/X11/keysymdef.h.

For characters that you don't use often enough to remember a key sequence anyway, you can use KCharSelect (or its Gnome equivalet GUCharMap). Or do what mathematicians are used to doing, which is type in \LaTeX.

I wrote this a long time ago. It's a script that sits between the user's input and an interactive program, and allows the input to be intercepted. I used it to escape to the shell to check filenames when running old Fortran programs that asked lots of questions. You could easily modify it to intercept particular inputs and sanitize them.

#!/usr/bin/perl

# shwrap.pl - Wrap any process for convenient escape to the shell.

use strict;
use warnings;

# Provide the executable to wrap as an argument
my $executable = shift;

my @escape_chars = ('#');             # Escape to shell with these chars
my $exit = 'bye';                     # Exit string for quick termination

open my $exe_fh, "|$executable @ARGV" or die "Cannot pipe to program $executable: $!";

# Set magic buffer autoflush on...
select((select($exe_fh), $| = 1)[0]);

# Accept input until the child process terminates or is terminated...
while ( 1 ) {
   chomp(my $input = <STDIN>);

   # End if we receive the special exit string...
   if ( $input =~ m/$exit/ ) {
      close $exe_fh;
      print "$0: Terminated child process...\n";
      exit;
   }

   foreach my $char ( @escape_chars ) {
      # Escape to the shell if the input starts with an escape character...
      if ( my ($command) = $input =~ m/^$char(.*)/ ) {
         system $command;
      }
      # Otherwise pass the input on to the executable...
      else {
         print $exe_fh "$input\n";
      }
   }
}

A simple example test program you can try it out on:

#!/usr/bin/perl

while (<>) {
   print "Got: $_";
}