[ragel-users] What is a good pattern for reentering parser and limiting repetitions?

Daniel Janzon janzon at gmail.com
Tue May 1 21:07:00 UTC 2012

Hi Guys,

I'd like to write a parser that can advance a little bit at a time as
input becomes available, with no need to look back. I came up
with the code below to test that. It's just a machine that must
read at least on 'a' and then at least one 'b'. Does it look like a
good pattern for this? I just save cs and set it againt when I come
back the next time. I guess the "nocs" option is not needed,
but does no harm either.

I also wonder what's a good way to limit repetitions. In the code
below, the buffer "result" could be overflowed. Either I build it
into the machine somehow (how?) or I need to check the buffer
limit for each byte in the action. I assume there must be a
standard way to do it but I couldn't find anything useful in the

All the best,
Daniel Janzon

#include <stdio.h>
#include <string.h>

  machine testreenter;
  write data;

struct parser
  char result[1024];
  int offset;
  int state;

int parse(char *str, int len, struct parser *parser)
  char *p=str, *pe=str+len;
  int cs;

    action character { parser->result[parser->offset++] = *fpc; }
    main := (('a'+ 'b'+) $character);
    write init nocs;

  cs = parser->state;

    write exec;
  parser->state = cs;

int main()
  char str1[] = "aaa";
  char str2[] = "bbb";
  struct parser parser;
  memset(&parser, 0, sizeof(struct parser));
  parser.state = testreenter_start;

  parse(str1, strlen(str1), &parser);
  parse(str2, strlen(str2), &parser);

  printf("result is '%s'\n", parser.result);

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