In the absence of any information about what you may be numbering, I can only point out the possibility of using a formatting switch. e.g. You could use SEQ fields to create the numbering.
{ SEQ MyNumber \# "'REQ_'0000" }
(The brackets { } are inserted with CTRL+F9 Save as Autotext or autocorrect for ease of re-insertion).
The only downside is that the SEQ fields do not update automatically. You can use F9, or you may find it more convenient to add a macro to the QAT (Quick Access Toolbar) to facilitate updating. The example, code at
http://www.gmayor.com/installing_macro.htm will do the job.