What is $1 in Perl?

Understanding $1 in Perl

In Perl, $1 is a special scalar variable that holds the content of the first capture group from the last successful regular expression match. It is one of several "match variables" ($1, $2, $3, etc.) that represent portions of a string matched within parentheses ().

When you use a regular expression with parentheses to capture parts of a string, Perl saves those captured substrings into these variables:

• $1 - First capture group
• $2 - Second capture group
• $3 - Third capture group
• ...and so forth

This feature is incredibly useful when you want to extract specific parts of a string.

Key Points About $1

• Scope: $1 is set after a successful match operator (=~) with capturing parentheses.
• Perl version: Available since early Perl versions; works consistently in all Perl 5 editions.
• Lifetime: $1 holds its value until next successful match or is overwritten.
• Use with caution: Avoid using $1 without an immediately preceding successful regex match; otherwise it retains stale data.
• Context: It's a scalar and should be used as such.

Example: Extracting a Number from a String

use strict;
use warnings;

my $string = "Order number: 12345";

if ($string =~ /number:\s+(\d+)/) {
    print "Captured number: $1\n";  # Prints: Captured number: 12345
} else {
    print "No match found.\n";
}

# Demonstrating multiple capture groups
my $date = "2024-06-20";

if ($date =~ /(\d{4})-(\d{2})-(\d{2})/) {
    print "Year: $1, Month: $2, Day: $3\n";  # Prints: Year: 2024, Month: 06, Day: 20
}

Explanation of the Code

In the first match, the pattern /number:\s+(\d+)/ searches for the word "number:" followed by one or more whitespace characters and then captures one or more digits (\d+) inside parentheses. These digits are stored in $1, which we then print.

In the second example, the regular expression has three capture groups corresponding to year, month, and day. Each is stored sequentially in $1, $2, and $3.

Common Pitfalls

• Using $1 without a prior successful regex match—this may print unexpected data or warnings.
• Assuming $1 stays the same across different matches—it gets overwritten on each successful match.
• Confusing numeric match variables ($1, $2) with capture variables such as %+ (named captures, Perl 5.10+ feature).

Summary

$1 is a powerful Perl variable that holds the contents of the first capturing group in the most recent successful regular expression match. It makes extracting specific fragments from strings straightforward and intuitive.