Python replace string

last modified January 9, 2023

Python replace string tutorial shows how to replace strings in Python.

There are several ways of replacing strings in Python:

replace method
re.sub method
translate method
string slicing and formatting

Python replace string with replace method

The replace method return a copys of the string with all occurrences of substring old replaced by new.

replace(old, new[, count])

The parameters are:

old − old substring to be replaced
new − new substring to replace old substring.
count − the optional count argument determines how many occurrences are replaced

replacing.py

#!/usr/bin/python

msg = "There is a fox in the forest. The fox has red fur."

msg2 = msg.replace('fox', 'wolf')

print(msg2)

In the example, both occurrences of word 'fox' are replaced with 'wolf'.

$ ./replacing.py
There is a wolf in the forest. The wolf has red fur.

Alternatively, we can use the str.replace method. It takes the string on which we do replacement as the first parameter.

replacing2.py

#!/usr/bin/python

msg = "There is a fox in the forest. The fox has red fur."

msg2 = str.replace(msg, 'fox', 'wolf')

print(msg2)

The example is equivalent to the previous one.

In the next example, we have a CSV string.

replacing3.py

#!/usr/bin/python

data = "1,2,3,4,5,6,7,8,9,10"

data2 = data.replace(',', '\n')

print(data2)

The replace each comma with a newline character.

$ ./replacing3.py
1
2
3
4
5
6
7
8
9
10
$ ./replacing3.py | awk '{ sum+=$1} END {print sum}'
55

Python replace first occurrence of string

The count parameter can be used to replace only the first occurrence of the given word.

replacing_first.py

#!/usr/bin/python

msg = "There is a fox in the forest. The fox has red fur."

msg2 = msg.replace('fox', 'wolf', 1)

print(msg2)

The example replaces the first occurrence of the word 'fox'.

$ ./replace_first.py
There is a wolf in the forest. The fox has red fur.

Python replace last occurrence of string

In the next example, we replace the last occurrence of word 'fox'.

replace_last.py

#!/usr/bin/python

msg = "There is a fox in the forest. The fox has red fur."

oword = 'fox'
nword = 'wolf'
n = len(nword)

idx = msg.rfind(oword)
idx2 = idx + n - 1

print(f'{msg[:idx]}{nword}{msg[idx2:]}')

We find the index of the last 'fox' word present in the message utilizing rfind method. We build a new string by omitting the old word an placing a new word there instead. We use string slicing and formatting operations.

$ ./replace_last.py
There is a fox in the forest. The wolf has red fur.

Python chaining of replace methods

It is possible to chain the replace methods to do multiple replacements.

chaining.py

#!/usr/bin/python

msg = "There is a fox in the forest. The fox has red fur."

msg2 = msg.replace('fox', 'wolf').replace('red', 'brown').replace('fur', 'legs')

print(msg2)

In the example, we perform three replacements.

$ ./chaining.py
There is a wolf in the forest. The wolf has brown legs.

Python replace characters with translate

The translate method allows to replace multiple characters specified in the dictionary.

translating.py

#!/usr/bin/python

msg = "There is a fox in the forest. The fox has red fur."

print(msg.translate(str.maketrans({'.': '!'})))

We replace the dot characters with the exclamation marks in the example.

$ ./translating.py
There is a fox in the forest! The fox has red fur!

Python replace string with re.sub

We can use regular expressions to replace strings.

re.sub(pattern, repl, string, count=0, flags=0)

The re.sub method returns the string obtained by replacing the leftmost non-overlapping occurrences of pattern in string by the replacement repl.

thermopylae.txt

The Battle of Thermopylae was fought between an alliance of Greek city-states,
led by King Leonidas of Sparta, and the Persian Empire of Xerxes I over the
course of three days, during the second Persian invasion of Greece.

We have a small text file.

replace_reg.py

#!/usr/bin/python

import re

filename = 'thermopylae.txt'

with open(filename) as f:

    text = f.read()


cleaned = re.sub('[\.,]', '', text)

words = set(cleaned.split())

for word in words:
    print(word)

We read the text file and use the re.sub method to remove the punctunation characters. We split the text into words and use the set function to get unique words.

cleaned = re.sub('[\.,]', '', text)

In our case, we only have a dot and comma punctunation characters in the file. We replace them with empty string thus removing them.

$ ./replace_reg.py
city-states
days
was
Empire
and
second
of
led
Battle
alliance
Greece
King
Persian
Leonidas
during
between
course
Thermopylae
Sparta
I
over
three
by
Xerxes
invasion
an
Greek
The
fought
the

In this tutorial, we have replaced strings in Python.

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