How to use the Perl command (with examples)

Perl is a programming language that is commonly used for text processing and automation tasks. It provides a command-line interface that allows users to execute Perl scripts and perform various operations on input data. In this article, we will explore several use cases of the Perl command with code examples, motivations, explanations, and example outputs.

Use case 1: Print lines from stdin matching regex1 and case insensitive regex2

Code:

perl -n -e 'print if m/regex1/ and m/regex2/i'

Motivation:

The motivation behind this use case is to filter and print lines from standard input (stdin) that match two specified regular expressions. By using the -n option, Perl reads input line by line and executes the following code for each line.

Explanation:

• -n: This option enables automatic line-by-line processing of the input.
• -e: This option allows us to provide a Perl script from the command line.
• print if m/regex1/ and m/regex2/i: This code prints the line if it matches both regex1 and regex2, with the /i modifier making the matching case-insensitive.

Example Output:

Suppose our input is the following three lines:

Hello World
This is a test
hELLo perl

If we execute the provided Perl command with regex1 set as Hello and regex2 set as perl, we will get the following output:

Hello World
hELLo perl

Use case 2: Say first match group, using a regexp, ignoring space in regex

Code:

perl -n -E 'say $1 if m/before (  group_regex  ) after/x'

Motivation:

The motivation behind this use case is to extract and print the first match group from each line of input data. By disregarding the spaces within the regular expression, the code becomes more readable and maintainable.

Explanation:

• -n: This option enables automatic line-by-line processing of the input.
• -E: This option allows us to execute code with features enabled as if a script was given to the command-line interpreter.
• say $1: This code prints the content of the first match group captured by the regular expression.
• m/before ( group_regex ) after/x: This regular expression captures the content between the strings “before” and “after”, regardless of the spaces surrounding the match group.

Example Output:

Suppose our input is the following three lines:

This is some text before matching group after.
Before   123  after.
No match group here.

If we execute the provided Perl command, we will get the following output:

matching group
123

Use case 3: In-place substitution of regex with backup

Code:

perl -i'.bak' -p -e 's/regex/replacement/g' path/to/files

Motivation:

The motivation behind this use case is to perform an in-place substitution of a regular expression with a replacement string in one or more files. By creating a backup file with the .bak extension, we can safely modify the original files while having a backup copy of the original content.

Explanation:

• -i'.bak': This option enables in-place editing of the specified files and creates a backup file with the .bak extension.
• -p: This option enables the automatic printing of each line after modification.
• -e: This option allows us to provide a Perl script from the command line.
• s/regex/replacement/g: This code performs a global substitution, replacing all occurrences of regex with replacement in each line of the input.

Example Output:

Suppose we have a file named example.txt with the following content:

This is a test text.
The regex is matching.
Replace regex with replacement.

If we execute the provided Perl command, replacing regex with expression, our file will be modified as follows:

This is a test text.
The expression is matching.
Replace expression with replacement.

Additionally, a backup file example.txt.bak will be created, containing the original content.

Use case 4: Use Perl’s inline documentation

Code:

perldoc perlrun ; perldoc module ; perldoc -f splice; perldoc -q perlfaq1

Motivation:

The motivation behind this use case is to access Perl’s inline documentation, which provides information about various aspects of the Perl language, modules, and frequently asked questions. By using the perldoc command followed by specific page names or options, we can retrieve relevant information without leaving the command-line interface.

Explanation:

• perldoc perlrun: This command displays the documentation for the Perl interpreter itself, explaining various command-line options and how to execute Perl code.
• perldoc module: This command displays the documentation for a specific Perl module, providing information on its usage, functions, and example code.
• perldoc -f splice: This command displays the documentation for a specific Perl function (splice in this case), explaining its purpose, syntax, and usage examples.
• perldoc -q perlfaq1: This command displays the documentation for a specific Perl FAQ section (perlfaq1 in this case), answering frequently asked questions about Perl programming.

Example Output:

Executing the provided command will display the inline documentation for the requested Perl pages. The output will consist of detailed explanations, code examples, and other relevant information related to the specific topics.

In conclusion, the Perl command provides a versatile and powerful way to process text and automate tasks. By utilizing its various options, users can filter and manipulate data, perform in-place substitutions, and access detailed documentation about the Perl language and its associated modules.