If you use the number() function here are the results you will get:
number("300") returns 300
number("abcde") returns error
number("301abcde100") returns 301
number("abde300") returns error
a better way to test for a number only vs an alphanumeric string is to use the format function like this:
format(300,"#") returns 300
format("300","#") returns 300
format("abcde","#") returns error
format("301abcde100") returns error
format("abde300") returns error
regards, Steve