Q. How do I determine if a value is a number?

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A. 1 (edit)

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+)$/) { print "is a decimal number\n" }
    if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
        { print "a C float\n" }
    

There are also some commonly used modules for the task. Scalar::Util (distributed with 5.8) provides access to perl's internal function "looks_like_number" for determining whether a variable looks like a number. Data::Types exports functions that validate data types using both the above and other regular expressions. Thirdly, there is "Regexp::Common" which has regular expressions to match various types of numbers. Those three modules are available from the CPAN.

If you're on a POSIX system, Perl supports the "POSIX::strtod" func- tion. 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 distribution) provides the "strtod" and "strtol" for converting strings to double and longs, respectively.