For example I have following string, which will be given as input from a user:

string str = "0-00:03"

Inside my contract I would like to check that:

  • str[1] should be "-"
  • str[4] should be ":"
  • and the remaining characters have to be numeric value.

This operation is easy to do C where with pointer I can iterate each character and check its char value. But I was not able to operate this inside my contract under Solidity language.


Just cast the string to bytes

   function testStr(string str) constant returns (bool){
        bytes memory b = bytes(str);
        if(b.length != 7) return false;
        for(uint i; i<7; i++){
                if(b[i] != 45) return false;
            else if (i==4){
                if(b[i] != 58) return false;
            else if(b[i] < 48 || b[i] > 57) return false;


        return true;
  • Thank you. Also for example when it pass the test, I would like to convert "03" into a uint32 variable. The thing come to my mind is: 0*10 + 3*1 <= 3. So can I do something like: (b[i] - 48) * 10 and cast result into uint32?@Tjaden Hess
    – alper
    Apr 10 '17 at 22:30
  • 1
    Sounds about right Apr 10 '17 at 23:00

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