Telling if characters in a string are numbers

i'm trying make rpn calculator out of arduino.  i have things split expression

code: [select]

1 2 3 4 + different number/ commands add stack. need differentiate between numbers (to add stack) , commands (to manipulate stack)
here's

code: [select]

#include <stacklist.h>
stacklist <int> stack;
string work;
string readstring;
void setup() {
 serial.begin(9600);
}

void loop() {

 while (serial.available()) {
   delay(10);
   char c = serial.read();  //gets 1 byte serial buffer
   readstring += c;
 }

 if (readstring.length() >0) {
   while(readstring.length()!=0) {
     readstring.trim();                                        //just in case
     readstring.concat(' ');                                   //so next function can work last argument
     work=readstring.substring(0,readstring.indexof(' '));     //finds data , operations
     
     
     if(work[0]=='0'||'1'||'2'||'3'||'4'||'5'||'6'||'7'||'8'||'9') {
       serial.println(work);        //output of equation
     }
     readstring=readstring.substring(readstring.indexof(' ')); //removes used argument
     readstring=readstring.trim();                             //removes white space @ beginning , end
   }
 }
}
any appriciated.
thanks,
isaac

this...

     if(work[0]=='0'||'1'||'2'||'3'||'4'||'5'||'6'||'7'||'8'||'9') {

...doesn't want or expect.  try instead...

     if ( (work[0] >= '0') && (work[0] <= '9') ) {

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