While working on a Perl script, I came across a problem when I wanted to save a part of a string into a new variable. In order to do so I decided to use a regular expression match and simply assign the match (group) to a new variable.
my $powerusage=$xml->{'POWER_SUPPLIES'}[0]->{'POWER_SUPPLY_SUMMARY'}[0]->{'PRESENT_POWER_READING'}[0]->{'VALUE'};
# At this point $powerusage value is: "167 Watts"
my $powerperf = $powerusage =~ m/(\d+) [Ww]atts/i;
print "power usage $powerusage ($powerperf)\n";
Basically the $powerusage variable is using a SimpleXML function to red a specific value from the XML document (saved as $xml). This works fine and results in a string value of "167 Watts".
The $powerperf variable should only contain the actual number of wattage, without any text. I tested the following regular expression m/(\d+) [Ww]atts/i on regex101 and everything seems correct:
However instead of the expected output "power usage 167 Watts (167)", $powerperf always showed a "1" as value: "power usage 167 Watts (1)".
However when the regex is directly printed to the script's output, without saving the value to a variable, the output showed the correct value:
my $powerusage=$xml->{'POWER_SUPPLIES'}[0]->{'POWER_SUPPLY_SUMMARY'}[0]->{'PRESENT_POWER_READING'}[0]->{'VALUE'};
my $powerperf = $powerusage =~ m/(\d+) [Ww]atts/i;
print "power usage $powerusage ($powerperf)\n";
print "power usage in numbers:\n";
print $powerusage =~ m/(\d+) [Ww]atts/i;
print "\n";
Output:
power usage 175 Watts (1)
power usage in numbers:
175
Research led to a question on stackoverflow where the same problem was discussed:
I get back a '1' as the value of $link. I assume this is because it found '1' match. But how do I save the content of the match instead?
Luckily user ikegami gave a solution where the first match can be saved into a variable by putting the variable itself into parentheses.
Note the /g to get all matches. Those can't possibly be put into a scalar. You need an array. If you just want the first match [...] note the parens (and the lack of now-useless /g). You need them to call m// in list context.
By applying this knowledge, the declaration of $powerperf was adjusted:
my $powerusage=$xml->{'POWER_SUPPLIES'}[0]->{'POWER_SUPPLY_SUMMARY'}[0]->{'PRESENT_POWER_READING'}[0]->{'VALUE'};
my ($powerperf) = $powerusage =~ m/(\d+) [Ww]atts/i;
print "power usage $powerusage ($powerperf)\n";
And the output is now correct:
power usage 158 Watts (158)
AWS Android Ansible Apache Apple Atlassian BSD Backup Bash Bluecoat CMS Chef Cloud Coding Consul Container Containers CouchDB DB DNS Database Databases Docker ELK Elasticsearch Filebeat FreeBSD Galera GlusterFS Grafana Graphics HAProxy HTML Hacks Hardware Icinga Icingaweb2 InfluxDB Internet Java KVM Kibana Kodi Kubernetes LTS LXC Linux Logstash Mac Macintosh Mail MariaDB Minio MongoDB Monitoring Multimedia MySQL NFS Nagios Network Nginx OSSEC OTRS Office PGSQL PHP Perl Personal PostgreSQL Postgres PowerDNS Proxmox Proxy Python Rancher SSL Samba Seafile Security Shell SmartOS Solaris Surveillance SystemD TLS Tomcat Ubuntu Unix VMWare VMware Varnish Virtualization Windows Wireless Wordpress Wyse ZFS Zoneminder