how can i prove a string does not contain a number ?

how can i prove a string does not contain a number ?

for example,

string s1="1-23" // there is a negative sign inside....so s1 does not hold number

string s2 = "12 " // this also does not contain a number as ther is a space at last

string s3 = "123"; // this holds a number

for this example string s1,s2 does not contain a number but string s3 contain a number (i.e 123).

how can i prove ?

atoi() function wont be helpful in this case bcoz they dont care delimeters.

however how can solve this ?