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Regular Expressions

by perlguru22 (Acolyte)
on Nov 15, 2012 at 01:06 UTC ( #1003909=perlquestion: print w/ replies, xml ) Need Help??
perlguru22 has asked for the wisdom of the Perl Monks concerning the following question:

I was having some trouble with this program. The user is going to enter a temperature in Fahrenheit or Celsius. The user should be allowed to enter numbers that may have a leading - or + and maybe integers or floats. Something like this: 32F, 32f, 32 F, +0.4F, -23C, -80.78 c, 98.6f I need to use a regular expression that will allow me to use what the user inputs in order to convert Fahrenheit into Celsius and vice verse. I have this so far.
Print "Enter a temperature.\n"; chomp ($temp = <STDIN>); while ($temp != 'q') { push @temp,$temp; print "Enter another temperature.\n"; chomp ($temp=<STDIN>); } print "Goobye see you next time.\n";

Comment on Regular Expressions
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Re: Regular Expressions
by toolic (Chancellor) on Nov 15, 2012 at 01:25 UTC
    I don't think you nedd a nasty regex:
    use warnings; use strict; use Scalar::Util qw(looks_like_number); while (<DATA>) { chomp; my $temp = $_; my $unit = lc chop $temp; if (looks_like_number($temp)) { print "temp = $temp : unit = $unit\n"; } } __DATA__ 55F 77c foo +0.4F

Re: Regular Expressions
by davido (Archbishop) on Nov 15, 2012 at 01:28 UTC

    I don't like using a regex as my only determination that I've gotten a number from the user. I'd rather let Perl tell me with 'Scalar::Util::looks_like_number'. It's more reliable. In that case, you're mostly interested in ascertaining whether you're looking at Fahrenheit or Celsius, and whether the input came in an expected format. This will do just that.

    use strict; use warnings; use Scalar::Util qw( looks_like_number ); # ..... if( $temp =~ m/^([0-9.+-]+)\s*([cf])/i and looks_like_number( $1 ) ) { my( $number, $system ) = ( $1, lc $2 ); # Do whatever you want here; # $number contains the temp, # and $system contains 'c' or 'f'. } else { die "Invalid number format."; }

    You'll notice the regex isn't too particular about what constitutes a number. It allows characters that are reasonable for a number to contain, but doesn't bother with whether the number makes sense. That's the job of "looks_like_number()". That tells us whether Perl thinks it can use the input as a number. We're using the regex just for capturing of the chunks of the input that we think should have a number, and a measurement system.


    Dave

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