How to delete lines matching a pattern with perl one-liner?

How to Delete Lines Matching a Pattern with a Perl One-liner

In Perl, deleting lines that match a pattern is typically done by processing input line-by-line and selectively printing only those lines that do not match the pattern. The common command-line switches are:

• -n: wraps the code in a loop over each input line but does not print automatically
• -p: same as -n, but prints each line after the code block runs

To remove lines matching a pattern, you can:

• Use print unless /pattern/ with -n, e.g. perl -ne 'print unless /error/' file.txt
• Use next if /pattern/; with -p, e.g. perl -pe 'next if /error/' file.txt

However, in sandbox environments without file or standard input access, you must simulate input lines inside the Perl script itself.

Runnable Perl Example (Self-contained, No File or Input Needed)

This example stores sample lines in an array and prints only those which do not contain the substring error. It runs directly with perl - and shows how to filter lines in Perl without external input:

use strict;
use warnings;

# Sample input lines
my @lines = (
    "System running smoothly\n",
    "Warning: error detected\n",
    "All checks passed\n",
    "Fatal error encountered\n",
);

foreach my $line (@lines) {
    # Print lines that do NOT match 'error' (case-sensitive)
    print $line unless $line =~ /error/;
}

This script uses:

• @lines array to simulate reading multiple lines
• foreach loop to iterate through each line
• print unless $line =~ /error/ to skip lines containing the pattern

Perl Concepts and Gotchas

• Sigils: $ for scalars (e.g., $line), @ for arrays (e.g., @lines), reflecting variable context
• Default variable $_: In Perl one-liners, each line is placed in $_ automatically, enabling concise pattern matching like print unless /pattern/.
• Context: The -n switch provides a loop over input without printing; -p adds automatic printing at loop end.
• TMTOWTDI ("There's more than one way to do it"): You can use either explicit print with conditional or next to skip lines.
• Regex case sensitivity: Use /pattern/i to match ignoring case.
• Common pitfall: Trying to run one-liner code without providing input files or input streams causes errors (as your environment lacks those).

Summary

• To filter lines from files or stdin, use Perl one-liners with -n or -p and conditional printing/skipping.
• In restricted sandbox contexts without file or stdin access, simulate input with an array inside a Perl script.
• Learn Perl’s loop switches (-n, -p) and the default $_ variable for idiomatic text processing.