Python replace string
last modified July 8, 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
#!/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.
#!/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.
#!/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.
#!/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'.
#!/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.
#!/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.
#!/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.
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.
#!/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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