Using find and replace text in python

Question

I'm trying to modify certain lines present in a file. I'm searching for a text and replacing it. For example, in the following code, I'm replacing vR33_ALAN with vR33_ALAN*c.

Here's my code for a test case

lines = ['x  = vR32_ALEX - vR33_ALAN; \n',
 'y = vR33_ALAN; \n']

text_to_search = 'vR33_ALAN'
replacement_text = 'vR33_ALAN*c'
for line in lines:
    print(line.replace(text_to_search, replacement_text), end='')

I could succeed in performing the above task. I want to add one more check before replacing the string that matches text_to_search.

I want to replace text_to_search with replacement_text only if a minus - is NOT present proceeding text_to_search.

Example, The output that I obtain is

x  = vR32_ALEX - vR33_ALAN*c;
y = vR33_ALAN*c;

Desired Output:

x  = vR32_ALEX - vR33_ALAN;
y = vR33_ALAN*c;

I'm not sure how to achieve the above. Any suggestions?

this is a good case for using regular expressions. I'm not an expert on them, but you should definitely google regular expressions and use the "re" module to do this. — bravosierra99
– bravosierra99, Commented Feb 28, 2019 at 4:08

blhsing · Accepted Answer · 2019-02-28 04:50:08Z

2

You can use re.sub with a negative lookbehind pattern:

import re
lines = ['x  = vR32_ALEX - vR33_ALAN; \n',
 'y = vR33_ALAN; \n']
for line in lines:
    print(re.sub(r'(?<!- )vR33_ALAN', 'vR33_ALAN*c', line), end='')

This outputs:

x  = vR32_ALEX - vR33_ALAN; 
y = vR33_ALAN*c;

answered Feb 28, 2019 at 4:50

blhsing

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2 Comments

Natasha Over a year ago

When I remove the space after - i.e. - vR33_ALAN; the output obtained is x = vR32_ALEX -vR33_ALAN*c ;. The desired output is x = vR32_ALEX - vR33_ALAN; . Therefore, I made a small change re.sub(r'(?<!-) and removed the space after -. I could obtain the output. But, how can we check for both cases? i.e irrespective of whether a space is present or not present after the minus sign , I want the output to be x = vR32_ALEX - vR33_ALAN;

Natasha Over a year ago

I tried using or print(re.sub(r'(?<!- |?<!-)vR33_ALAN', 'vR33_ALAN*c', line), end='').But didn't succeed.Any suggestions?

TavoGLC · Accepted Answer · 2019-02-28 04:50:47Z

You can perform that with and without the use of regular expressions. You can simply add the '-'character to text_to_searchand use find to search for the new string

lines = ['x  = vR32_ALEX - vR33_ALAN; \n',
 'y = vR33_ALAN; \n']

text_to_search = 'vR33_ALAN'
replacement_text = 'vR33_ALAN*c'

for line in lines:
  if line.find('- '+text_to_search)!=-1:
    print(line)
  else:
    print(line.replace(text_to_search, replacement_text),end='')

Or you can use the re module as suggested, for that you have to generate a pattern to search, as you're looking for '-'or add text_to_search as before. The (.*)is to specify that it doesn't matter the characters before and after the pattern.

import re 
lines = ['x  = vR32_ALEX - vR33_ALAN; \n',
 'y = vR33_ALAN; \n']

for line in lines:
  if re.match('(.*)'+' - '+'(.*)',line):
    print(line)
  else:
    print(line.replace(text_to_search, replacement_text),end='')

The pattern '(.*)'+' - '+text_to_search+'(.*)'should also work. Hope it helps

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