Script to count filled fields in adobe acrobat form

I am trying to make a form in which it is required to count the number of filled fields .

The fillable fields are named as FIELD1, FIELD2, FIELD3 ..... ........................FIELD 27, FIELD 28.

Below is the javascript i am using to count the filled fields.

var counter = 0;

for (i=1; i<=28; i++) {

    if (this.getField("FIELD" + i).value!=="") counter++;

}

if (counter>0) {

    event.value = counter;

} else event.value = "";

Everything is working fine.. except for that it also counts the fields in which spaces are put......... which i don't want.. i want if only texts or numbers are input into field then only count the fields.. .