This seems to be the best way to do this, but I kepts a few other experiments in scratchpad.

scratchpad/ansi_parser_normal.rl

@@ -0,0 +1,82 @@
+#include <fmt/core.h>
+#include <string_view>
+#include <charconv>
+#include "dbc.hpp"
+#include <SFML/Graphics.hpp>
+
+using namespace fmt;
+
+%%{
+  machine foo;
+
+  action tstart {
+    start = fpc;
+  }
+
+  action number {
+    auto [ptr, ec] = std::from_chars(start, fpc, value);
+    dbc::check(ec == std::errc(), "error in number parsing");
+    println("NUM: {}", value);
+  }
+
+  action color24b {
+    println("color24b");
+  }
+
+  action bg { fgcolor = false; }
+  action fg { fgcolor = true; }
+
+  action any {
+    println("ANY: {}:{}", int(fc), fc);
+  }
+
+  action red { color.r = value; }
+  action blue { color.g = value; }
+  action green { color.b = value; }
+
+  action start { value = 0; }
+
+  action end {
+    println("fg? {}, sf:Color: {},{},{},{}", fgcolor, color.r, color.g, color.b, color.a);
+  }
+
+  start = 0x1B "[";
+  fg = "38;" %fg;
+  bg = "48;" %bg;
+  reset = ("39" | "49");
+  num = digit+ >tstart %number;
+  color256 = "5;";
+  color24b = "2;";
+
+  ansi = (
+      start %start
+      (
+        reset |
+        (fg|bg) color24b num %red ";" num %blue ";" num %green %color24b
+      ) "m" %end
+  );
+
+  other = (any+ @any -- 0x1B)*;
+
+  main := (other :> ansi)**;
+}%%
+
+%% write data;
+
+bool parse_ansi(std::string_view codes) {
+  const char *start = NULL;
+  int cs = 0;
+  uint8_t value = 0;
+  const char *p = codes.data();
+  const char *pe = p + codes.size();
+  const char *eof = pe;
+  bool fgcolor = false;
+  sf::Color color;
+
+  %% write init;
+  %% write exec;
+
+  print("PROCESSED {} CHARS of {}: {}", p - codes.data(), codes.size(), p);
+
+  return true;
+}

scratchpad/ansi_parser_scanner.rl

@@ -0,0 +1,97 @@
+#include <fmt/core.h>
+#include <string_view>
+#include <charconv>
+#include "dbc.hpp"
+
+using namespace fmt;
+
+%%{
+  machine ansi_parser;
+
+  action tstart {
+    start = fpc;
+  }
+
+  action number {
+    auto [ptr, ec] = std::from_chars(start, fpc, value);
+    dbc::check(ec == std::errc(), "error in number parsing");
+    println("NUM: {}", value);
+  }
+
+  action color256 {
+    println("COLOR256");
+  }
+
+  action color24b {
+    println("COLOR24B");
+  }
+
+  action colorSingle {
+    println("SINGLE");
+  }
+
+  action colorBasic {
+    println("BASIC");
+  }
+
+  action bg {
+    println("BG");
+  }
+
+  action fg {
+    println("FG");
+  }
+
+  action any {
+    println("ANY: {}:{}", int(fc), fc);
+  }
+
+  action start {
+    println("START");
+  }
+
+  action end {
+    println("END");
+  }
+
+  ESC = 0x1B;
+  start = ESC "[";
+  fg = "38;" %fg;
+  bg = "48;" %bg;
+  color256 = "5;" %color256;
+  color24b = "2;" %color24b;
+  num = (digit+) >tstart %number;
+
+  ansi := |*
+      start => start;
+      (fg|bg) color256 num ";" num => { println("256 doit"); };
+      (fg|bg) color24b num ";" num ";" num => { println("24b doit"); };
+      num ";" num => colorBasic;
+      num => colorSingle;
+      "m" => { fgoto main; };
+  *|;
+
+  main := (any @any -- ESC)* ESC @{ fhold; fgoto ansi; };
+
+}%%
+
+%% write data;
+
+bool parse_ansi(std::string_view codes) {
+  const char *start = NULL;
+  int cs = 0;
+  size_t value = 0;
+  const char *p = codes.data();
+  const char *pe = p + codes.size();
+  const char *eof = pe;
+  const char *ts = p;
+  const char *te = p;
+  int act = 0;
+
+  %% write init;
+  %% write exec;
+
+  print("PROCESSED {} CHARS of {}", p - codes.data(), codes.size());
+
+  return true;
+}