I need restrict entries to a particular format With there entries start

I need restrict entries to a particular format With there entries starting in numeric and ending in alphanumeric. But i need to keep the string length variable. how can i do that?

I need the users to fill in description of goods in the following format.

(Two Digit Numerical Value) (Multiplication Sign - /Character/) (Two Digit Numerical Value) (fixed string /Conts/)

(Variable length Numeric Value) (Alphanumeric String of variable length).

For Eg:

01 X 20 Conts 300 Bags LDPE

10 X 40 Conts 50000 ctns black pepper.

If you use Input mask feature the string length will be fixed. 

As a possible solution you can use separate fields, 3 numeric fields and 1 text box field, and then display a result in the separate field. 

Here is my demo form: https://form.jotform.com/72763963879983 

Feel free to test it and clone it. 

I used Form Calculation widget to display the final format.

I also set the limit for the first two number fields. 

Will this solution work for you?