PERL ONE-LINERS - REAL WORLD EXAMPLES

These are battle-tested patterns for common tasks.

Log File Processing

Extract timestamps from logs:

perl -ne 'print "$1\n" if /(\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2})/' app.log

Count lines containing "error":

perl -ne '$c++ if /error/i; END { print "$c\n" }' app.log

Extract unique IP addresses:

perl -ne 'print "$1\n" if /(\d+\.\d+\.\d+\.\d+)/' access.log | sort -u

CSV Processing

Print second column of a CSV:

perl -F',' -ane 'print "$F[1]\n"' data.csv

Skip the header line:

perl -F',' -ane 'print "$F[1]\n" if $. > 1' data.csv

Reorder columns (print columns 3, 1, 2):

perl -F',' -ane 'print "$F[2],$F[0],$F[1]\n"' data.csv

JSON Wrangling

Extract a specific field (crude but works):

perl -ne 'print "$1\n" if /"name":\s*"([^"]+)"/' data.json

For serious JSON work, use jq. But for quick extracts, this works.

Bulk Text Transformation

Convert Windows line endings to Unix:

perl -i -pe 's/\r\n/\n/' *.txt

Convert Unix to Windows:

perl -i -pe 's/\n/\r\n/' *.txt

Remove blank lines:

perl -ne 'print unless /^$/' file.txt

Collapse multiple blank lines to one:

perl -00 -pe '' file.txt

The -00 flag enables "paragraph mode."

Working with Base64

This comes up constantly with AI APIs.

Remove newlines from base64:

cat image.b64 | perl -pe 's/\s//g'

Add newlines every 76 characters (standard format):

perl -pe 's/(.{76})/$1\n/g' raw.b64

Extracting Data from AI Responses

Given a response like:

NUDE_CHEST: NO
NUDE_LOWER: NO
WEAPON: YES
CONFIDENCE: 0.87

Extract the WEAPON value:

perl -ne 'print "$1\n" if /WEAPON:\s*(\w+)/' response.txt

Extract all YES values:

perl -ne 'print "$1\n" if /(\w+):\s*YES/' response.txt

Parse confidence as a number:

perl -ne 'print "$1\n" if /CONFIDENCE:\s*(\d+\.\d+)/' response.txt

Batch File Content Replacement

Fix a typo across all files:

perl -i -pe 's/teh/the/g' *.txt

Update a version number:

perl -i -pe 's/version: 1\.0/version: 1.1/' config.yml

Comment out lines matching a pattern:

perl -i -pe 's/^(.*DEBUG.*)$/# $1/' code.py

Reformatting Data

Convert space-separated to CSV:

perl -pe 's/\s+/,/g' data.txt

Convert tabs to spaces:

perl -pe 's/\t/    /g' file.txt

Remove duplicate spaces:

perl -pe 's/  +/ /g' file.txt

Extracting URLs

Pull all URLs from a file:

perl -ne 'print "$1\n" while /(https?:\/\/[^\s"<>]+)/g' page.html

The while loop finds all matches, not just the first.

Extracting Email Addresses

perl -ne 'print "$1\n" while /([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})/g' file.txt

Processing stdin from Other Commands

Combine with curl:

curl -s https://example.com | perl -ne 'print "$1\n" if /<title>([^<]+)</'

Combine with find:

find . -name "*.log" | perl -ne 'chomp; print "Found: $_\n"'

Combine with ack:

ack -o "\d+" | perl -ne '$sum += $_; END { print "Total: $sum\n" }'

Multi-line Processing

Read entire file as one string:

perl -0777 -pe 's/old/new/g' file.txt

The -0777 flag slurps the whole file. Useful for patterns spanning lines.

Remove C-style comments:

perl -0777 -pe 's|/\*.*?\*/||gs' code.c

BEGIN and END Blocks

Set up variables before processing:

perl -ne 'BEGIN { $sum = 0 } $sum += $_; END { print $sum }' nums.txt

Print headers:

perl -pe 'BEGIN { print "=== OUTPUT ===\n" }' file.txt

The -l Flag

Automatically chomp input and add newlines to output:

perl -lne 'print uc' file.txt

This is cleaner than manually handling newlines.

Debugging One-Liners

Add print statements to see what's happening:

perl -ne 'print "LINE: $_"; print "MATCH: $1\n" if /(\d+)/' file.txt

Use Data::Dumper for complex data:

perl -MData::Dumper -ne '@F = split /,/; print Dumper(\@F)' data.csv

Converting to a Script

When a one-liner gets too complex, make it a script:

#!/usr/bin/env perl
use strict;
use warnings;

while (<>) {
    s/old/new/g;
    print;
}

Save as script.pl, chmod +x, and run on any file.

Speed Tips

For huge files, Perl one-liners are fast. But:

1. Avoid capturing groups if you don't need them 2. Use index() for literal string matching 3. Pre-compile regex with qr// for repeated matches

Example with qr//:

perl -ne 'BEGIN { $re = qr/pattern/ } print if /$re/' huge.txt