How to introspect regexes in the Perl API

Question

I'm working on some code that needs to serialize Perl regexes, including any regex flags. Only a subset of flags are supported, so I need to detect when unsupported flags like /u are in the regex object.

The current version of the code does this:

static void serialize_regex_flags(buffer *buf, SV *sv) {
  char flags[] = {0,0,0,0,0,0};
  unsigned int i = 0, f = 0;
  STRLEN string_length;
  char *string = SvPV(sv, string_length);

Then manually processes string char-by-char to find flags.

The problem here is that the stringification of regex flags changed (I think in Perl 5.14) from e.g. (?i-xsm:foo) to (?^i:foo), which makes parsing a pain.

I could check the version of perl, or just write the parser to handle both cases, but something tells me there must be a superior method of introspection available.

ikegami · Accepted Answer · 2012-08-08T15:32:55.073

In Perl, you'd use re::regexp_pattern.

 my $re = qr/foo/i;
 my ($pat, $mods) = re::regexp_pattern($re);
 say $pat;   # foo
 say $mods;  # i

As you can see from the source of regexp_pattern, there's no function in the API to obtain that information, so I recommend that you call that function too from XS too.

perlcall covers calling Perl functions from C. I came up with the following untested code:

/* Calls re::regexp_pattern to extract the pattern
 * and flags from a compiled regex.
 *
 * When re isn't a compiled regex, returns false,
 * and *pat_ptr and *flags_ptr are set to NULL.
 *
 * The caller must free() *pat_ptr and *flags_ptr.
 */

static int regexp_pattern(char ** pat_ptr, char ** flags_ptr, SV * re) {
   dSP;
   int count;
   ENTER;
   SAVETMPS;
   PUSHMARK(SP);
   XPUSHs(re);
   PUTBACK;
   count = call_pv("re::regexp_pattern", G_ARRAY);
   SPAGAIN;

   if (count == 2) {
      /* Pop last one first. */
      SV * flags_sv = POPs;
      SV * pat_sv   = POPs;

      /* XXX Assumes no NUL in pattern */
      char * pat   = SvPVutf8_nolen(pat_sv); 
      char * flags = SvPVutf8_nolen(flags_sv);

      *pat_ptr   = strdup(pat);
      *flags_ptr = strdup(flags);
   } else {
      *pat_ptr   = NULL;
      *flags_ptr = NULL;
   }

   PUTBACK;
   FREETMPS;
   LEAVE;

   return *pat_ptr != NULL;
}

Usage:

SV * re = ...;

char * pat;
char * flags;
regexp_pattern(&pat, &flags, re);

Thanks, @ikegami. I was able to get what I needed with your C code as a starting point. One thing to note is that the return values have to be popped in reverse order (so `flags_sv` comes off first instead of second.) — friedo, Aug 07 '12 at 20:06

score 3 · Answer 2 · answered Aug 07 '12 at 18:19

use Data::Dump::Streamer ':util';
my ($pattern, $flags) = regex( qr/foo/i );
print "pattern: $pattern, flags: $flags\n";
# pattern: foo, flags: i

But if you are trying to restrict more recent features, you have a lot more work to do than just checking for /u.

How to introspect regexes in the Perl API

2 Answers2

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