function validateVoterID(ths) {
var inputvalues = $(ths).val();
var regex = /^([a-zA-Z]){3}([0-9]){7}?$/g;
if (!regex.test(inputvalues)) {
$(ths).val("");
$(ths).parent().append('<span class="error">Invalid Voter ID Number</span>');
return regex.test(inputvalues);
} else {
$(ths).parent().find(".error").remove();
return true;
}
}
<input type="text" name="voter_IDno" class="form-control" onchange="return validateVoterID(this)" placeholder="Enter Voter ID Number" maxlength="10">