[Chicago-talk] detecting whether a scalar is number or a string.

Jay Strauss me at heyjay.com
Mon Jul 7 06:35:11 PDT 2008

Hi, as Andy pointed out its in the FAQ.  Which you can do by:

perldoc -q number

(but quoted here for your reading enjoyment)

  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+)$/) { print "is a decimal number\n" }
       if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                            { 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" 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 distribution) provides the
    "strtod" and "strtol" for converting strings to double and longs,


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