Executing regex in C code

Hi,
I'm trying to execute the equivalent to '~' with regexp from within C code.
According to pg_operator
<https://github.com/postgres/postgres/blob/c7b8998ebbf310a156aa38022555a24d98fdbfb4/src/include/catalog/pg_operator.h>this
operator for type text corresponds to textregexeq:

DATA(insert OID = 641 ( "~" PGNSP PGUID b f f 25 25 16 0 642 textregexeq
regexeqsel regexeqjoinsel ));
DESCR("matches regular expression, case-sensitive");

I've tried to execute it from the C code:

text * s = cstring_to_text("aa");
text * p = cstring_to_text("a");
return DatumGetBool(DirectFunctionCall2(textregexeq, PointerGetDatum(s),
PointerGetDatum(p)));

But I'm getting an error with collate:

ERROR: could not determine which collation to use for regular expression
HINT: Use the COLLATE clause to set the collation explicitly.

Any hints on how to proceed? Of course if I do select 'aa'~'a'; from psql
it works like a charm.

Thanks in advance.