Python re.sub Examples Edit Cheat Sheet

Syntax

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

Simple Examples

result = re.sub('abc',  '',    input)           # Delete pattern abc
result = re.sub('abc',  'def', input)           # Replace pattern abc -> def
result = re.sub(r'\s+', ' ',   input)           # Eliminate duplicate whitespaces using wildcards
result = re.sub('abc(def)ghi', r'\1', input)    # Replace a string with a part of itself

Note: Take care to always prefix patterns containing \ escapes with raw strings (by adding an r in front of the string). Otherwise the \ is used as an escape sequence and the regex won’t work.

Advance Usage

Replacement Function

Instead of a replacement string you can provide a function performing dynamic replacements based on the match string like this:

def my_replace(m):
    if <some condition>:
        return <replacement variant 1>
    return <replacement variant 2>

result = re.sub("\w+", my_replace, input)

Count Replacements

When you want to know how many replacements did happen use re.subn() instead of re.sub()

print ('Result: ', result[0])
print ('Replacements: ', result[1])

Using Backreferences

Numeric Backreferences

To use back reference define capture groups using () and reference to those using \1, \2, and so on. Do not forget the ‘r’ prefix on the back reference string, otherwise \1 will be interpreted as a character.

result = re.sub("(\d+) (\w+)", r"\2 \1")

Named Backreferences

For named backreferences define a named capture group (?) and reference using \g. Again ensure to add the 'r' prefix on the backreference string.

result = re.sub("(?<number>\d+) (?<word>\w+)", r"\g<word> \g<number>")