perl - Detect letter where number was expected -

how can detect , show error if put letter , program expects number?

a regexp match makes easy. searching character isn't numeral or arithmetic symbol:

if ( $input =~ /[^0-9+*/-]/ ) {   print "incorrect character detected!\n" } 

literally letter:

if ( $input =~ /[a-za-z]/ ) {   print "incorrect character detected!\n" }