avp_subst: parse errors

Hello experts,

can anyone help me to find a syntax error at the following lines in my
cfg-file:

   avp_write("$hdr[Proxy-Authorization]","i:100");
   avp_subst("i:100/i:101","/(?<=username=")\w+(?=")//");
   avp_print();

The middle line (330) with the "avp_subst" function causes the following
parse errors:

0(6799) parse error (330,19-20): syntax error
0(6799) parse error (330,19-20): bad command: missing ';'?

What might be wrong?

Regards
Franz