Found this old code

IsControlCharacterFSM.c

#include "IsControlCharacterFSM.h"

// This is a state machine to determine whether or not the passed character is part of a 
// TELNET control sequence.
// It must be called for every character read in order to manage state appropriately.
// Credit where credit is due: 
// Ryan Rawson (ryan@netidea.com) wrote this for FreeTrade. some time around 2000.
bool IsControlCharacterFSM(char ch)
{
   static enum {norm,riac,rddww,rsb,rsiac} state = norm;
     
   bool IsControlCharacter = true;
   
   switch(state) {
      case norm:
         if(ch == IAC) 
            state = riac;
         else
            IsControlCharacter = false;
      break;
         
      case riac:
         switch(ch) {
            case IAC:
               state = norm;
            break;
            
            case DO:
            case DONT:
            case WILL:
            case WONT:
               state = rddww;
            break;
            
            case SB:
               state = rsb;
            break;
            
            default:
               state = norm;
         }
      break;
         
      case rddww:
         state = norm;
      break;
      
      case rsb:
         if(ch == IAC)
            state = rsiac;
      break;
      
      case rsiac:
         if(ch == SE)
            state = norm;
         else
            state = rsb;
      break;
   }
   
   return IsControlCharacter;
}

IsControlCharacterFSM.h

#if !defined(ISCONTROLCHARACTERFSM_H_INCLUDED)
#define ISCONTROLCHARACTERFSM_H_INCLUDED

bool IsControlCharacterFSM(char ch);

/* as per defined by various TELNET RFC's */
/* these are from the basic TELNET RFC */
#define IAC  ((char)255)
#define DO   ((char)253)
#define WILL ((char)251)
#define WONT ((char)252)
#define DONT ((char)254)
#define SB  ((char)250)
#define SE  ((char)240)
/* from the TOPT-ECHO RFC */
#define ECHO ((char)1)
/* from the TOPT-SGA RFC */
#define SUPPRESSGA ((char)3)
/* from the TOPT-LINE RFC */
#define LINEMODE ((char)34)

#endif // !defined(ISCONTROLCHARACTERFSM_H_INCLUDED)