Input Validation Help

It depends what you mean by numeric.
perldoc -q "How do I determine whether a scalar is a number/whole/integer/float?"
Found in /Library/Perl/pods/perlfaq4.pod
       How do I determine whether a scalar is a number/whole/integer/float?

               Assuming that you don't care about IEEE notations like "NaN" or
               "Infinity", you probably just want to use a regular expression.

                  if (/\D/)            { print "has nondigits\n" }
                  if (/^\d+$/)         { print "is a whole number\n" }
                  if (/^-?\d+$/)       { print "is an integer\n" }
                  if (/^[+-]?\d+$/)    { print "is a +/- integer\n" }
                  if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
                  if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal numbe
r\n" }
                  if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                                       { print "a C float\n" }

               You can also use the Data::Types module on the CPAN, which
               exports functions that validate data types using these and
               other regular expressions.

               If you're on a POSIX system, Perl's supports the "POSIX::str-
               tod" function.  Its semantics are somewhat cumbersome, so
               here's a "getnum" wrapper function for more convenient access.
               This function takes a string and returns the number it found,
               or "undef" for input that isn't a C float.  The "is_numeric"
               function is a front end to "getnum" if you just want to say,
               ``Is this a float?''

                   sub getnum {
                       use POSIX qw(strtod);
                       my $str = shift;
                       $str =~ s/^\s+//;
                       $str =~ s/\s+$//;
                       $! = 0;
                       my($num, $unparsed) = strtod($str);
                       if (($str eq '') || ($unparsed != 0) || $!) {
                           return undef;
                       } else {
                           return $num;
                       }
                   }

                   sub is_numeric { defined getnum($_[0]) }

               Or you could check out the String::Scanf module on the CPAN
               instead. The POSIX module (part of the standard Perl distribu-
               tion) provides the "strtod" and "strtol" for converting strings
               to double and longs, respectively.

I want to ensure that the input contains only numbers 0-9

I do not want to allow any other characters including  a decimal point.

$input might contain a trailing \n if you read it as user input

chomp($input) ;
die "$input is not numeric\n" if $input ~ /\D/;

Ozo, for regular expressions, is "matching all characters to be numeric" faster or "matching even one character non-numeric" faster?? I believe the apporach given by me will be faster...is that true??

Manav

$input =~ /\D/; may be faster, but it will succeed for $input = ''; which I thought should be considered non-numeric.  
Although " I want to ensure that the input contains only numbers 0-9" seems to allow input containg no characters.