Python, Perl: Find/Replace Strings in a File

By Xah Lee. Date: . Last updated: .

This pages shows a simple example of Python and Perl scripts for find and replace strings in a file.

Python

# -*- coding: utf-8 -*-
# python

import sys

nn = len(sys.argv)

if nn != 5:
     print "Error: number of arguments does not match. Syntax should be: %s searchString replaceString inputFile outputFile" % sys.argv[0]
else:
     searchStr = sys.argv[1]
     replaceStr = sys.argv[2]
     inputFile = open(sys.argv[3])
     outputFile = open(sys.argv[4],'w')

     for thisLine in inputFile:
         outputFile.write(thisLine.replace(searchStr,replaceStr))

     outputFile.close()
     inputFile.close()

Save this code as find_replace.py and run it like this:

python find_replace.py findStr replacementStr inputFile outputFile

The sys.argv is from the module sys.

The value of sys.argv[0] is the calling script's name itself.

Note the idiom for var in file_object. It will loop thru the lines in file.

This code was originally based from Python Cookbook 2nd edition, by Alex Martelli, David Ascher, page 121. Python Cookbook

Perl

Here is a similar code in Perl.

# -*- coding: utf-8 -*-
# perl

if (scalar @ARGV != 4) {die "Error: number of arguments does not match. Syntax should be: $0 searchString replaceString inputFile outputFile\n"}

$searchStr=$ARGV[0];
$replaceStr=$ARGV[1];
$inputFile = $ARGV[2];
$outputFile = $ARGV[3];

open(F1, "<$inputFile") or die "Error: $!";
open(F2, ">$outputFile") or die "Error: $!";

while ($line = <F1>) {
     $line =~ s/$searchStr/$replaceStr/g;
     print F2 "$line";
}

close(F1) or die "Error: $!";
close(F2) or die "Error: $!";

For a full-featured script that does find-replace in Perl, see: Perl: Find Replace String Pairs in Directory.

If you have a question, put $5 at patreon and message me.

Python by Example

  1. Python Basics
  2. Print Version String
  3. Builtin Help
  4. Quote String
  5. String Operations
  6. String Methods
  7. Format String
  8. True, False
  9. if then else
  10. for, while, Loops
  11. List Basics
  12. Loop Thru List
  13. Map Function to List
  14. List Comprehension
  15. List Methods
  16. Dictionary
  17. Loop Thru Dict
  18. Dict Methods
  19. Function
  20. Class
  21. List Modules
  22. Write a Module
  23. Unicode 🐍

Regex

  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. Manipulate Path
  4. Process Unicode
  5. Convert File Encoding
  6. Find Replace in dir
  7. Find Replace by Regex
  8. Count Word Frequency

Web

  1. Send Email
  2. GET Web Page
  3. Web Crawler
  4. HTTP POST
  5. Check Page Load Size
  6. Thumbnail Generation

Misc

  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Complex Numbers

Advanced

  1. Sort
  2. Copy Nested List
  3. Tuple vs List
  4. Sets, Union, Intersection
  5. Closure in Python 2
  6. Decorator
  7. Append String in Loop
  8. Timing f timeit
  9. Keyword Arg Default Value Unstable