re.c:478
static void
rb_reg_raise(s, len, err, re)
    const char *s;
    long len;
    const char *err;
    VALUE re;
{
    VALUE desc = rb_reg_desc(s, len, re);

    if (ruby_in_compile)
	rb_compile_error("%s: %s", err, RSTRING(desc)->ptr);
    else
	rb_raise(rb_eRegexpError, "%s: %s", err, RSTRING(desc)->ptr);
}
