How to use LENGTH function In transformation file

Hi,

Currently we are working on NW BPC 10.0

I am trying to validate below transformation file, but I am getting error near length function.

Can anyone help me how to use length function or any other option.

Here ACCOUNT is coming from flatfile with value similarly like 123,it should be converted into AA_00000123 if length of ACCOUNT is equal to 3.

Thanks in Advance.

Regards

Anitha