Perl Regex How To Print The Matched String

by StackCamp Team 43 views

In Perl, regular expressions are a powerful tool for pattern matching and text manipulation. This article will guide you through the process of extracting and printing specific strings from a larger text using Perl's regex capabilities. We'll focus on matching strings followed by " ETN : " and printing the first occurrence before the string "data reached". This task is common in log file analysis, data extraction, and other text-processing scenarios. By the end of this guide, you'll have a clear understanding of how to use Perl's regular expressions to achieve this efficiently.

Before diving into the code, let's break down the problem. We need to:

  1. Identify the pattern we want to match, which is a string followed by " ETN : ".
  2. Extract the matched string.
  3. Locate the first occurrence of the matched string before "data reached".
  4. Print the extracted string.

This involves crafting a regular expression that accurately captures the desired pattern and using Perl's regex functions to find and extract the match. Additionally, we need to ensure that we stop searching once we encounter "data reached" to avoid printing incorrect matches.

The key to solving this problem lies in creating the right regular expression. Here’s how we can approach it:

  • We want to match a string followed by " ETN : ". The string can contain various characters, so we'll use a character class and quantifiers to match it.
  • The pattern should be flexible enough to handle different string formats, such as those containing letters, numbers, and special characters.
  • We need to use capturing groups to extract the specific part of the matched text that we're interested in.

Considering these points, a suitable regular expression would be (.*?) ETN : . Let's break this down:

  • (.*?): This is the capturing group. It matches any character (.) zero or more times (*), but as few times as possible (?). The non-greedy quantifier ? is crucial here to ensure we capture only the string immediately before " ETN : " and not a larger chunk of text.
  • ETN :: This matches the literal string " ETN : ", which is the delimiter we're looking for.

This regex effectively targets the strings we want to extract. For instance, if our text contains name1/name2 ETN : value, the capturing group (.*?) will match name1/name2.

Now that we have our regular expression, let's write the Perl code to extract and print the matched string. Here’s a complete example:

#!/usr/bin/perl

use strict;
use warnings;

my $text = "Some text name1/name2 ETN : somevalue other text data reached name3/name4 ETN : anothervalue";
my $pattern = '(.*?) ETN : ';
my $data_reached = 'data reached';

if ($text =~ /($pattern)/) {
    my $matched_string = $1;
    my $data_reached_pos = index($text, $data_reached);

    if ($data_reached_pos == -1 || index($text, $matched_string) < $data_reached_pos) {
        print "Matched string: $matched_string\n";
    }
}

Let's walk through the code:

  1. #!/usr/bin/perl: This is the shebang line, specifying the interpreter for the script.
  2. use strict; and use warnings;: These pragmas enforce good coding practices by requiring explicit variable declarations and enabling warnings for potential issues.
  3. my $text = ...;: This line defines the input text where we'll search for the pattern. In this example, it includes multiple occurrences of the pattern and the "data reached" string.
  4. my $pattern = '(.*?) ETN : ';: This line defines the regular expression pattern we discussed earlier.
  5. my $data_reached = 'data reached';: This line defines the string that marks the end of our search scope.
  6. if ($text =~ /($pattern)/) { ... }: This is the core of the script. It uses the =~ operator to test whether the $text matches the $pattern. The parentheses around $pattern in the regex create a capturing group, allowing us to extract the matched substring.
  7. my $matched_string = $1;: If a match is found, this line assigns the content of the first capturing group (i.e., the string before " ETN : ") to the $matched_string variable. $1 is a special variable that holds the content of the first capturing group.
  8. my $data_reached_pos = index($text, $data_reached);: This line finds the position of the string "data reached" in the $text. The index function returns the position of the first occurrence of a substring within a string, or -1 if the substring is not found.
  9. if ($data_reached_pos == -1 || index($text, $matched_string) < $data_reached_pos) { ... }: This conditional statement checks whether we should print the matched string. It has two conditions:
    • $data_reached_pos == -1: If "data reached" is not found in the text, we print the matched string.
    • index($text, $matched_string) < $data_reached_pos: If the matched string occurs before "data reached", we print it. This ensures that we only print the first occurrence of the matched string before the "data reached" marker.
  10. `print