[OpenSIPS-Users] Runtime avp substitution

[Paul Smith]

How can I do a regular expression substitution on an avp where the 
pattern is loaded from the user subsrciption or usr_preference table?

For example I need to limit callerid presented to a pre-determined range 
owned by my customer, and perform substitution eg add a prefix.  If a 
UAC sets P-Preferred-identify to 1234 and I have a usr_preference to 
match any 4 digit caller starting with 1, and prefix with 789 then 
callerid should be set to 7891234

I have in mind something like:

In my database I set the expression for  $avp(s:expcallerid) for this 
user to "/1.{3}/789\1/i"
then in my dialplan I want to match and substitute the 
Preferred-Identity to form the $avp(s:callerid) and then set 
P-Asserted-Identity

xlog("dialled number = $rU");
xlog("callerid barring requested by device = $avp(s:clir)");
xlog("callerid preferred set by device = $pU");
xlog("callerid expression for this user = $avp(s:expcallerid)");
xlog("default callerid for this user = $avp(s:defcallerid)");

$avp(s:callerid)=$avp(s:defcallerid);
if($avp(s:clir)){
       # user has restricted callerid
        $avp(s:callerid)="Anonymous";
} else if($pU) {
      # UAC device has set preferred callerid
      $avp(s:callerid)=pU;
      if(avp_subst("$avp(s:callerid)",$avp(s:expcallerid)) ){
            xlog("preferred callerid matched callerid expression");
      } else {
           xlog("preferred callerid rejected, using default");
           $avp(s:callerid)=$avp(s:defcallerid);
      }
} else {
      #use default callerid
      $avp(s:callerid)=$avp(s:defcallerid);
}

I get the error
Sep 22 12:30:21 [6253] CRITICAL:core:yyerror: parse error in config 
file, line 518, column 44-45: bad arguments for command <avp_subst>

which I think means that avp_subst is expecting a static regular 
expression in the script, whereas  I want a user variable as the regular 
expression.

Is there another way to look at the problem? Have I missed something 
obvious (again)?

Thanks
Paul