How to count occurrences of a pattern using perl -ne?

Counting Occurrences of a Pattern Using perl -ne

The perl -ne one-liner is excellent for reading input line-by-line and processing it without printing each line automatically. To count the total occurrences of a pattern across all input lines, you accumulate matches in a variable and print the result in an END block, which runs after all input is processed.

The common idiom $count += () = /pattern/g; uses a clever trick:

• The regex match /pattern/g in list context returns all matches.
• The empty list assignment () = ... forces list context on the match.
• The scalar assignment to $count adds the number of matches found on that line.

This approach ensures you count every match on every line efficiently.

Fixed Runnable Example: Count “cat” From Standard Input

This corrected example can be copy-pasted and run directly with perl -. It reads from standard input until EOF (Ctrl+D / Ctrl+Z), counts all non-overlapping occurrences of cat (case-sensitive), then prints the total:

use strict;
use warnings;

my $count = 0;

while ( <> ) {
    $count += () = /cat/g;
}

END {
    print "Total occurrences of \"cat\": $count\n";
}

You can run it interactively:

perl - 
I have a cat.
My cat likes to catch mice.
catcatcat
<Ctrl+D>

Output will be:

Total occurrences of "cat": 6

Explanation of Key Perl Concepts

• -n wraps your code in while (<>) { ... }, reading input line-by-line.
• $/ is the input record separator (default newline).
• () = /pattern/g forces list context to count all non-overlapping matches globally.
• END blocks run after input processing finishes, useful to output totals.
• use strict and use warnings help avoid common bugs.

Common Gotchas

• Always accumulate counts using the $count += () = /pattern/g; idiom, not just /pattern/g.
• Regexes are case-sensitive by default; use /pattern/gi for case-insensitive matching.
• Global regex matches count non-overlapping occurrences only; overlapping matches require more complex solutions.
• When running via perl -, ensure your complete script is passed on stdin or use proper command-line quoting for one-liners.

Summary

Counting pattern occurrences over input in Perl is most robust using perl -ne with an accumulator variable and printing in an END block. The () = /pattern/g trick provides the count per line efficiently. The corrected code example above runs error-free and demonstrates these best practices clearly.