Assuming an OCR’d book or other text file that has line numbers embedded in file neat.py:

1 # cool program
2 import sys
3 def howneat():
4     sys.exit('Thanks for visiting')

Remove the line numbers by typing in Terminal:

perl -pe 's/^[ t]+d+//' neat.py  > neater.py

Regular expression:

^[ t]+d+

\^

beginning of the line

[ t]+

match space or tab (one or more)

d+

match one or more digits

-p

implicitly loop over (read each line of) the file neat.py.

-e

enable PCRE

And we replace with nothing. This leaves alone the indentation of the code (relevant for Python).

I could have used the -i flag to edit in place, but I instead redirected STDOUT to neater.py in case I made a mistake (inputting the wrong file, for example).