How to receive only numeric( decimal content) data from string input.

In brief,  I need to give </= 12 digits data as input string like "989789856999".

 If I give "1abc23d7bad7657" as input string then need to pop up a message as invalid data,( i.e, if any char,special symboles (expect numaric data) it should be pop up msg like invalid data)

If i give "999999999999" , "912345678912", "123456789123","1233456","99999123"
then it is "valid data" .