Advanced Regular Expression Patterns

Lookahead Guard

A lookahead can require nearby text without consuming it as part of the match.

lookahead Lookahead is useful when the program needs to confirm what comes next while keeping the matched text focused on the part being classified.

Lookahead Guard

file
lookahead_guard.pl
Replay: real traced execution (multi-file project)
use strict;
use warnings;

my $file = "report.csv";
my ($stem) = $file =~ /^([a-z]+)(?=\.csv$)/;
$stem = defined $stem ? $stem : "none";
my $csv = $stem ne "none" ? "yes" : "no";

print "file=$file\n";
print "stem=$stem\n";
print "csv=$csv\n";

use strict;
use warnings;

my $file = "image.png";
my ($stem) = $file =~ /^([a-z]+)(?=\.csv$)/;
$stem = defined $stem ? $stem : "none";
my $csv = $stem ne "none" ? "yes" : "no";

print "file=$file\n";
print "stem=$stem\n";
print "csv=$csv\n";

use strict;
use warnings;

my $file = "archive.csv.bak";
my ($stem) = $file =~ /^([a-z]+)(?=\.csv$)/;
$stem = defined $stem ? $stem : "none";
my $csv = $stem ne "none" ? "yes" : "no";

print "file=$file\n";
print "stem=$stem\n";
print "csv=$csv\n";

  1. $file ← report.csv, $stem ← report, $csv ← yes

    4my $file→ report.csv = "report.csv"; #@file="image.png", "archive.csv.bak"5my ($stem→ report) = $filereport.csv =~ /^([a-z]+)(?=\.csv$)/;6$stemreport = defined $stem ? $stem : "none";7my $csv→ yes = $stemreport ne "none" ? "yes" : "no";89print "file=$filereport.csv\n";10print "stem=$stemreport\n";11print "csv=$csvyes\n";
    outputfile=report.csv
stem=report
csv=yes

  1. $file ← image.png, $stem ← (empty), $csv ← no

    4my $file→ image.png = "image.png";5my ($stem→ (empty)) = $fileimage.png =~ /^([a-z]+)(?=\.csv$)/;6$stem→ none = defined $stem ? $stem : "none";7my $csv→ no = $stemnone ne "none" ? "yes" : "no";89print "file=$fileimage.png\n";10print "stem=$stemnone\n";11print "csv=$csvno\n";
    outputfile=image.png
stem=none
csv=no

  1. $file ← archive.csv.bak, $stem ← (empty), $csv ← no

    4my $file→ archive.csv.bak = "archive.csv.bak";5my ($stem→ (empty)) = $filearchive.csv.bak =~ /^([a-z]+)(?=\.csv$)/;6$stem→ none = defined $stem ? $stem : "none";7my $csv→ no = $stemnone ne "none" ? "yes" : "no";89print "file=$filearchive.csv.bak\n";10print "stem=$stemnone\n";11print "csv=$csvno\n";
    outputfile=archive.csv.bak
stem=none
csv=no