How to print matching groups from regex with perl -ne?

When using perl -ne from the command line to print regex capture groups, the main issues typically arise from quoting and argument placement. The code snippet must be enclosed in quotes, and input files (like input.txt) should be listed after the quoted script, not inside it.

In the error you saw, the shell tried to parse input.txt as part of the quoted Perl code, causing a syntax error. Always separate the Perl code from input filenames.

How -ne works

• -n wraps your code in while(<>){...}, processing input line-by-line.
• -e tells Perl to execute the given snippet.
• Your regex capture groups () fill $1, $2, etc.
• You must explicitly print these variables; Perl does not auto-print with -n.
• Use single quotes in Unix shells to protect Perl variables and regex characters.

Correct Example of Printing Matching Groups

use strict;
use warnings;

# Sample lines read from STDIN or files passed as arguments.
# Prints first two colon-separated fields on each line.
while (my $line = <>) {
    chomp $line;
    if ($line =~ /^([^:]+):([^:]+):/) {
        print "$1 and $2\n";
    }
}

# To run as a one-liner from shell:
# perl -ne 'print "$1 and $2\n" if /^([^:]+):([^:]+):/' input.txt

Explanation

The regex /^([^:]+):([^:]+):/ captures two groups of characters separated by colons at the start of the line. These are stored in $1 and $2. We explicitly print them inside the loop.

In the one-liner, perl -ne 'print "$1 and $2\n" if /^([^:]+):([^:]+):/' input.txt, the code snippet is single-quoted to avoid shell interpolation, and input.txt comes after the quotes as an argument, not inside.

Common Pitfalls

• Incorrect quoting: use single quotes around your Perl snippet in Unix shells.
• Including input filenames inside the quotes causes syntax errors.
• Not printing the capture variables explicitly.
• Regex doesn't match input format (check your anchors and pattern).
• For Windows cmd.exe, double quotes and escaping rules differ.

Summary

To print matching groups from a regex with perl -ne, capture your groups with parentheses, access them via $1, $2, etc., within the loop, and print explicitly. Proper quoting and argument order are essential to avoid syntax errors.