The code became a tad long. Much longer than I initially thought it would be.
Your conditions aren't too clear, I think. What I implemented (in crappy pseudocode) is:
procedure cur_char // (a, b, c, or d)
if next_char is lowercase_vowel
print replacement (cur_char) // replaces ABCD by PQRS
else
if next_char is uppercase_X
override_replacement // changes replacement of ABC to LMN
end
print replacement (cur_char) // possibly overriden because of X
print "x"
end
end
which is more or less what the code does in an over simplified way.
With that code, the following substitutions occur:
I used expl3 for the job, otherwise the code would be much longer. I defined a macro \niranjan_define_active:Nn which you should use, for example, like this:
\niranjan_define_active:Nn A { \__niranjan_process:NN A P }
Another pair of macros \activateall and \deactivateall controls when the replacement behaviour should happen. Ideally you want this replacement active in a group to avoid chaos.
The code can be extented to handle other types of substitutions if you want to. The \__niranjan_define_charset_conditional:Nn allows you to define a conditional function to check if the next character belongs to a given set of characters.
However the code only works for characters with catcode 11. I didn't do any verification whatsoever for other catcodes.
I must say I doubt this code will simply work for the Devanagari script. But, as you wanted an example for latin characters, here it is:
\documentclass{article}
\usepackage{expl3}
\ExplSyntaxOn
% Main code
\cs_new_protected:Npn \__niranjan_process:NN #1#2
{
\cs_set_eq:NN \l__niranjan_curr_char #1
\cs_set_eq:NN \l__niranjan_replacement_char #2
\niranjan_deactivate_all:
\peek_after:Nw \__niranjan_process_aux:
}
\cs_new_protected:Npn \__niranjan_process_aux:
{
\__niranjan_if_lower_vowel:NTF \l_peek_token
{
\l__niranjan_replacement_char
\__niranjan_rescan_token:w
}
{
\__niranjan_if_upper_X:NT \l_peek_token
{ \__niranjan_followed_by_X: }
\l__niranjan_replacement_char
x
\__niranjan_rescan_token:w
}
}
\cs_new_protected:Npn \__niranjan_followed_by_X:
{
\__niranjan_if_upper_ABC:NT \l__niranjan_curr_char
{
\exp_args:Nf \__niranjan_replace_char:n
{ \__niranjan_get_char:N \l__niranjan_curr_char }
}
}
\cs_new:Npn \__niranjan_replace_char:n #1
{
\exp_last_unbraced:NNf \cs_set_eq:NN \l__niranjan_replacement_char
\str_case:nnF {#1}
{
{ A } { L }
{ B } { M }
{ C } { N }
}
{#1}
}
\cs_new_protected:Npn \__niranjan_rescan_token:w
{
\peek_N_type:TF
{ \__niranjan_rescan_token:Nw }
{ \niranjan_activate_all: }
}
\cs_new_protected:Npn \__niranjan_rescan_token:Nw #1
{
\niranjan_activate_all:
\tl_rescan:nn { } {#1}
}
% Checking for following charset
\cs_set:Npn \__niranjan_tmp:w #1
{
\cs_new_protected:Npn \__niranjan_define_charset_conditional:Nn ##1 ##2
{
\prg_new_protected_conditional:Npnn ##1 ####1 { T, F, TF }
{
\exp_last_unbraced:No \__niranjan_if_charset:wn
\token_to_meaning:N ####1 #1 \q_nil \q_stop {##2}
}
}
\cs_new_protected:Npn \__niranjan_if_charset:wn ##1 #1 ##2##3 \q_stop ##4
{
\quark_if_nil:NTF ##2
{ \prg_return_false: }
{
\str_if_in:nnTF {##4} {##2}
{ \prg_return_true: }
{ \prg_return_false: }
}
}
\cs_new:Npn \__niranjan_get_char:N ##1
{ \exp_last_unbraced:No \__niranjan_get_char:w \token_to_meaning:N ##1 }
\cs_new:Npn \__niranjan_get_char:w #1 ##1 { ##1 }
}
\use:x { \exp_not:N \__niranjan_tmp:w { \tl_to_str:n { the~letter~ } } }
\__niranjan_define_charset_conditional:Nn \__niranjan_if_lower_vowel:N { aeiou }
\__niranjan_define_charset_conditional:Nn \__niranjan_if_upper_X:N { X }
\__niranjan_define_charset_conditional:Nn \__niranjan_if_upper_ABC:N { ABC }
% Setting active chars
\tl_new:N \g__niranjan_chars_tl
\cs_new_protected:Npn \niranjan_define_active:Nn #1#2
{
\cs_gset_protected:cpn { __niranjan_active_letter_#1: } {#2}
\char_set_active_eq:Nc #1 { __niranjan_active_letter_#1: }
\tl_gput_right:Nn \g__niranjan_chars_tl { #1 }
}
\cs_new_protected:Npn \niranjan_activate_all:
{ \tl_map_function:NN \g__niranjan_chars_tl \char_set_catcode_active:N }
\cs_new_protected:Npn \niranjan_deactivate_all:
{ \tl_map_function:NN \g__niranjan_chars_tl \char_set_catcode_letter:N }
\cs_new_eq:NN \activateall \niranjan_activate_all:
\cs_new_eq:NN \deactivateall \niranjan_deactivate_all:
\niranjan_define_active:Nn A { \__niranjan_process:NN A P }
\niranjan_define_active:Nn B { \__niranjan_process:NN B Q }
\niranjan_define_active:Nn C { \__niranjan_process:NN C R }
\niranjan_define_active:Nn D { \__niranjan_process:NN D S }
\ExplSyntaxOff
\begin{document}
\ttfamily
AaBeCiDoAu ->
\activateall
AaBeCiDoAu
\deactivateall
ABCDA ->
\activateall
ABCDA
\deactivateall
AXBXCXDXAX ->
\activateall
AXBXCXDXAX
\deactivateall
\end{document}
\tikz@handle. There are lots of nested conditionals which check for each possible letter and the ones that follow it. – Phelype Oleinik Jul 02 '19 at 11:15\meaningpart you would then use your character remapping. – siracusa Jul 03 '19 at 18:47