Differentiate a word/letter and number in a string

[panadol]

I have one string from user. Let say,
"The deviceId is 2"....but user also can give input like this..
"The deviceId is a". I just need to know how to differentiate a word/letter and number in the string, any help will be so much appreciate.

[LB]

http://www.cplusplus.com/reference/cctype/