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

Answer 1

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);

Answer 2

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.