Perl Regex - Getting Text Before and After Match - regex

Question

I am parsing a tab delimited file line by line:
Root rootrank 1 Bacteria domain .72 Firmicutes phylum 1 Clostridia class 1 etc.
=
while (my $line = <$fh>) {
chomp($line);
}
On every line, I want to capture the 1st entry before and after a particular match. For example, for the match phylum, I want to capture the entries Firmicutes and 1. For the match domain, I want to capture the entries Bacteria and .72. How would I write the regex to do this?
Sidenote: I can't simply split the line by tab into an array and use the index because sometimes a category is missing or there are extra categories, and that causes the entries to be shifted by one or two indices. And I want to avoid writing blocks of if statements.

Answer 1

You can still split the input, then map the words to indices, and use than use the indices corresponding to the matches to extract the neighbouring cells:
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
my #matches = qw( phylum domain );
while (<>) {
chomp;
my #cells = split /\t/;
my %indices;
#indices{ #cells } = 0 .. $#cells;
for my $match (#matches) {
if (defined( my $index = $indices{$match} )) {
say join "\t", #cells[ $index - 1 .. $index + 1 ];
}
}
}
What's missing:
You should handle the case when $index == 0 or $index == $#cells.
You should handle the case where some words are repeated in one line.

Answer 2

my $file = "file2.txt";
open my $fh, '<', $file or die "Unable to Open the file $file for reading: $!\n";
while (my $line = <$fh>) {
chomp $line;
while ($line =~ /(\w+)\s+(\w+)\s+(\.?\d+)/g) {
my ($before, $match, $after) = ($1, $2, $3);
print "Before: $before Match: $match After: $after\n";
}
}

Answer 3

You can just simply use the following regex to capture the words before and after of a matched word:
(?<LSH>[\w.]+)[\s\t](?<MATCH>.*?)[\s\t](?<RHS>[\w.]+)
see demo / explanation

Answer 4

You could do:
#!/usr/bin/perl
use Modern::Perl;
my #words = qw(phylum domain);
while(<DATA>) {
chomp;
for my $word (#words) {
my ($before, $after) = $_ =~ /(\S+)(?:\t\Q$word\E\t)(\S+)/i;
say "word: $word\tbefore: $before\tafter: $after";
}
}
__DATA__
Root rootrank 1 Bacteria domain .72 Firmicutes phylum 1 Clostridia class 1 etc.
Output:
word: phylum before: Firmicutes after: 1
word: domain before: Bacteria after: .72