%needs a fontloader 2017-07-28 or later

% !Mode:: "TeX:UTF-8:Main"
%needs a fontloader 2017-07-28 or later
%substitutions at word boundaries

\documentclass{article}

\usepackage{luacode,fontspec}
\begin{luacode*}
fonts.handlers.otf.addfeature {
name = "test-a",
type = "chainsubstitution",
lookups = {
{
type = "substitution",
data = {
["a"] = "A",
["b"] = "B",
["c"] = "C",
["d"] = "D",
},
},
{
type = "ligature",
data = {
['1'] = { "a", "b" },
['2'] = { "c", "d" },
},
},
},
data = {
rules = {
{
before = { { " ", 0xFFFC } },
current = { { "a" }, { "b" } },
lookups = { 2 },
},
{
current = { { "c" }, { "d" } },
after = { { 0xFFFC, " " } },
lookups = { 2 },
},
{
current = { { "a" } },
after = { { "b" } },
lookups = { 1 },
},
{
current = { { "c" } },
after = { { "d" } },
lookups = { 1 },
},
},
},
}

fonts.handlers.otf.addfeature {
name = "test-b",
type = "chainsubstitution",
lookups = {
{
type = "ligature",
data = {
['1'] = { "a", "b" },
['2'] = { "c", "d" },
},
},
},
data = {
rules = {
{
-- the space is redundant as 0xFFFC contains it
before = { { " ", 0xFFFC } },
current = { { "a" }, { "b" } },
lookups = { 1 },
},
{
current = { { "c" }, { "d" } },
-- the space is redundant as 0xFFFC contains it
after = { { 0xFFFC, " " } },
lookups = { 1 },
},
},
},
}

fonts.handlers.otf.addfeature {
name = "test-c",
type = "chainsubstitution",
lookups = {
{
type = "ligature",
data = {
['1'] = { "a", "b" },
['2'] = { "c", "d" },
},
},
},
data = {
rules = {
{
before = { { " " } },
current = { { "a" }, { "b" } },
lookups = { 1 },
},
{
current = { { "c" }, { "d" } },
after = { { " " } },
lookups = { 1 },
},
},
},
}
\end{luacode*}

\setmainfont{Arial}[RawFeature=+test-a;]

\begin{document}
Substitutions at word boundaries (spaces):

ab ab abxcd abxcdcdcd

expected output:

1 1 1x2 1xCdCd2

\end{document}