If you have a text file and want to search and replace a part of a matched pattern, how would you do it with perl one-liners, sed or python?

For example:


match with [0-9]{6},[0-9]{2}.

Change , to ..

2 Answers 2


You can use numbered sub groups. i.e:





Example in Python:

import re
inp = '653433,78'
out = re.sub(r'([0-9]{6}),([0-9]{2})', r'\1.\2', inp)
print out

This would give you:

  • For using number variables in replacement strings, named reference syntax can be used: re.sub(pat, r'\g<1>' + f'{my_number}', str)
    – aksh1618
    Apr 25, 2019 at 18:38

You can capture the matched part around the pattern you want to replace and then reuse it in the replace statement.

cat yourfile | perl -p -e "s/([0-9]{6}),([0-9]{2})/\\1.\\2/g"

A more complex solution with lookahead and lookbacks can also be used (but this will not support wildcards such as * and + for your pattern)

cat yourfile | perl -p -e "s/(?<=[0-9]{6}),(?=[0-9]{2})/./g"

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