Found my old LEL code and got the shell working, so tomorrow I'll try to make it layout some gui element.

lel.rl

@@ -0,0 +1,87 @@
+#include "lel.hpp"
+#include <fmt/core.h>
+
+%%{
+  machine LELParser;
+  alphtype char;
+
+  action token {tk = input.substr(start - begin, fpc - start); }
+
+  action col { col(); }
+  action ltab { ltab(); }
+  action valign { valign(fc); }
+  action id { id(input.substr(start - begin, fpc - start)); }
+  action row { row(); }
+  action align { align(fc); }
+  action setwidth { setwidth(std::stoi(tk)); }
+  action setheight { setheight(std::stoi(tk)); }
+  action expand { expand(); }
+
+  col = "|" $col;
+  ltab = "[" $ltab;
+  rtab = "]" $row;
+  valign = ("^" | ".") $valign;
+  expand = "*" $expand;
+  halign = ("<" | ">") $align;
+  number =  digit+ >{ start = fpc; } %token;
+  setw = ("(" number %setwidth ("," number %setheight)? ")") ;
+  modifiers = (expand | valign | halign | setw);
+  id = modifiers* ((alpha | '_')+ :>> (alnum | '_')*) >{start = fpc;} %id;
+  row = space* ltab space* id space* (col space* id)* space* rtab space*;
+
+  main := row+;
+}%%
+
+%% write data;
+
+bool LELParser::parse(std::string input) {
+  int cs = 0;
+  const char *start = nullptr;
+  const char *begin = input.data();
+  const char *p = input.data();
+  const char *pe = p + input.size();
+  std::string tk;
+
+  %% write init;
+  %% write exec;
+
+  bool good = pe - p == 0;
+
+  return good;
+}
+
+void LELParser::col() {
+  fmt::println("col");
+}
+
+void LELParser::ltab() {
+  fmt::println("ltab");
+}
+
+void LELParser::valign(char dir) {
+  fmt::println("valign: {}", dir);
+}
+
+void LELParser::align(char dir) {
+  fmt::println("align {}", dir);
+}
+
+void LELParser::id(std::string name) {
+  fmt::println("id: {}", name);
+}
+
+void LELParser::row() {
+  fmt::println("row");
+}
+
+void LELParser::setwidth(int width) {
+  fmt::println("setwidth: {}", width);
+}
+
+void LELParser::setheight(int height) {
+  fmt::println("setheight: {}", height);
+}
+
+void LELParser::expand() {
+  fmt::println("expand");
+}