Perl string search

Question

I am reading data from a CSV file that contains a quantities with the units attached. I need to separate the value from the units.

The values I read in could have units of either millivolts mV or just volts V. If the string in $splitter[0] is 1.987mV. I want to separate that into two values, 1.987 and mV.

$splitter[0] =~ /(.*)([mV])/;
print "$1 -- $2\n";

This outputs

1.987m -- V

If the units in $splitter[0] is V then it seems to be working.

Does anyone know why I'm not picking up the m?

Matt Jacob · Accepted Answer · 2015-11-03 21:38:21Z

2

You have no repetition after your character class, so you're asking for a single match of anything in that character class, i.e., an m or a V. There are many ways to skin this cat, though:

/^([\d.]+)(\D+)$/
/^([\d.]+)(\w+)$/
/^([^A-Za-z]+)(\w+)$/
/^([^A-Za-z]+)([A-Za-z]+)$/
/(.*)(mV|m)/
/(.*)(m?V)/

Some solutions above are more "correct" than others.

Also, you generally want to try to be as restrictive as possible with your regular expressions and match exactly what you mean. Try to stay away from . if you can help it, and prefer more specific patterns instead.

edited Nov 3, 2015 at 21:38

answered Nov 3, 2015 at 20:59

Matt Jacob

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3 Comments

Sobrique Over a year ago

That last- should it not be m?V?

Matt Jacob Over a year ago

D'oh! It's harder coming up with wrong solutions than I thought. ;-)

milnuts Over a year ago

Thanks, I went with option #2 above and it worked perfectly.

Borodin · Accepted Answer · 2015-11-03 22:55:08Z

As you have read, one reason your code is finding only V at the end of the string is because your character class matches only one character. [mV] matches either a single small m or a single capital V. To match more than one character you need a quantifier like [mV]+ which will match one or more characters, like m or V or mVm or mV etc.

The other reason is that you have a greedy match before it. .* will match zero or more of any character, so even if you fixed the quantifier on the units and wrote /(.*)([mV]+)/ you would still get 1.9876m and V because the dot is quite happy to match the m, leaving [mv]+ to match just V

Assuming the quantity is numeric, consisting of decimal digits and possibly a decimal point, and the units are always letters (including perhaps a Greek mu μ for micro) then you can split the value like this

use utf8;
use strict;
use warnings 'all';
use v5.10;

use open qw/ :std :encoding(UTF-8) /;

my @splitter = qw/ 1.987mV 442.0μH /;

for ( @splitter ) {

    my ($val, $units) = / ([0-9.]+) (\p{Letter}+) /x;

    say "$val ~ $units";
}

output

1.987 ~ mV
442.0 ~ μH

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