My turn to try:

--snip--
/*
 * escaped string interpreter
 *
 * gcc version
 *
 * plp 2007
 *
 * Convention:
 *
 * Input is a string in ASCIIZ suppied as argv[1]
 * Escape characters escape themselves: CHAR_ESC CHAR_ESC -> CHAR_ESC
 *
 * Example usage:
 *
 * ./escape 'abc \"\e "ABC\"def\"GHI\t"'
 *
 * Note that by convention an input consisting of a single escape char
 * '\' is illegal. The program does not detect this.
 *
 * compile with:
 *
 * gcc -Wall -g -o escape escape.c
 *
 */

#include <stdio.h>
#include <errno.h>
#include <err.h>

#define VERSION "0.0"
#define BANNER "escape.c " VERSION "\n"

#define CHAR_ESC    '\\'
#define CHAR_DQUOTE '"'
#define CHAR_CR     '\r'
#define CHAR_NL     '\n'
#define CHAR_TAB    '\t'

enum _interp_states { I_IDLE = 0, I_STRING, I_OTHER };

int main( int argc, char *argv[] )
{
  int Is = I_IDLE; 
  char *p;

  printf( BANNER );
  if(argc != 2) 
    err( 1, "usage: scape \"string\"\n" );

  // process argv[1]
  p = argv[1];

  if(!*(p+1)) { // special, length == 1
    
    printf( "short input '%c'\n", *p);   

  } else { // length >= 2
    
    while(*p++) { // now *p and *(p-1) exist and are not NUL
      
      // esc is always esc, other escs depend on context
      // this can be changed by moving this code inside the state machine
      if(*(p-1) == CHAR_ESC) {
        switch(*p) { 
          case CHAR_ESC:
            printf( "%c", CHAR_ESC );
            ++p;
            continue; // at while()
          default:
            ; // handled in state machine below
        }
      }

      // state machine, defines context (in string etc)
      switch( Is ) {
  
        case I_IDLE:
          if(*(p-1) == CHAR_ESC) {
            
            switch(*p) {
              case CHAR_DQUOTE:
                printf( "ESC-QUOTE\n" );
                ++p;
                break;
              default:
                printf( "ESC-OTHER: '%c'\n", *p );
                ++p;
                break;
            }
          
          } else {
            
            if(*(p-1) == CHAR_DQUOTE) { // string start quote
              Is = I_STRING;
              printf( "\nString: '" );
            } else {
              printf( "%c", *(p-1)); // ordinary char outside string
            }

          }

          break;

        case I_STRING:
            
          if(*(p-1) == CHAR_ESC) {
            
            switch(*p) { // inside string, escape various things
              case CHAR_DQUOTE:
                printf( "%c", CHAR_DQUOTE );
                break;
              case CHAR_CR:
                printf( "\r" );
                break;
              case CHAR_NL:
                printf( "\n" );
                break;
              case CHAR_TAB:
                printf( "\t" );
                break;

              default: // unknown escape, print error and cont.
                printf( "ESC-UNKNOWN: '%c'\n", *p );
                break;
            }
            ++p; // eat both escape and escaped
          
          } else {
          
            if(*(p-1) == CHAR_DQUOTE) { // string closing quote
              Is = I_IDLE;
              printf( "'\n" );
            } else {
              printf( "%c", *(p-1)); // ordinary char inside string
            }
          
          }

          break;
      }  
    }
  }

  printf( "\n\n*** Success. End.\n" );
  return( 0 );
}

// editor settings: set tabs to 2 spaces
// vim:ts=2:sw=2
--snap--

Peter 'state machines always win' P.


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