How to Efficiently Replace a Line Containing a Specific Word in a Text File?

Find and Replace a Specific Line in a Text File

Handling text files is a common task in many programming scenarios. When working with large files, it can be a challenge to identify and modify specific lines of text without knowing their exact line numbers. This article demonstrates an effective approach for replacing a line containing a particular word in a text file using PHP, regardless of its position in the file.

Approach 1: Handling Smaller Files in Memory

For smaller files that can be loaded entirely into memory, an efficient solution is to read all lines into an array and then apply a replacement function. The following code snippet walks through this process:

$data = file('myfile'); // read an array of lines

function replace_a_line($data) {
   if (stristr($data, 'certain word')) {
     return "replacement line!\n";
   }
   return $data;
}

$data = array_map('replace_a_line', $data);

file_put_contents('myfile', $data);

With PHP versions 5.3.0 and above, you can use lambda functions to simplify the replacement step:

$data = array_map(function($data) {
  return stristr($data,'certain word') ? "replacement line\n" : $data;
}, $data);

Approach 2: Handling Larger Files Efficiently

However, for larger files that may exceed available memory, a different strategy is needed. The following code example uses a streaming approach to read and replace specific lines while consuming minimal memory:

$reading = fopen('myfile', 'r');
$writing = fopen('myfile.tmp', 'w');

$replaced = false;

while (!feof($reading)) {
  $line = fgets($reading);
  if (stristr($line,'certain word')) {
    $line = "replacement line!\n";
    $replaced = true;
  }
  fputs($writing, $line);
}

fclose($reading); fclose($writing);

// Handle the replacement result
if ($replaced)
{
  rename('myfile.tmp', 'myfile');
} else {
  unlink('myfile.tmp');
}

In this scenario, if at least one line is replaced, the temporary file 'myfile.tmp' is renamed to 'myfile,' effectively overwriting the original file with the updated content. Otherwise, if no replacements were made, the temporary file is deleted.

Conclusion

These techniques provide efficient and flexible methods for finding and replacing specific lines in text files based on a given word. Whether dealing with smaller files that fit in memory or larger files requiring a streaming approach, these solutions offer practical approaches for handling this common text file manipulation task.

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