string operation/regex - find and replace

Question

I am trying to replace each word after . in the txt file below:

line1
line2
field: [orders.cancelled,orders.delivered,orders.reached
orders.pickup,orders.time]
some line
some line

I have a dictionary:

   d = {'cancelled':'cancelled_at', 'deliver':'xxx'}

I am running the following code. However, I am getting the results for partial match i.e

I see the new file has the following words

field: [orders.cancelled_at, orders.xxxed ..........

here from the word delivered the program is still replacing the first 7 words(deliver) and adding 'ed' in the end. I am not sure why

with open('list.txt', 'r') as g:
    text = g.read()
    for k in d:
        before = f'.{k}'
        after = f'.{d[k]}
        #print(before)
        #print(after)
        text = text.replace(before, after)
        #print(text)

with open('new_list.txt', 'w') as w:
    w.write(text)

I also tried this one and I get the same results

import re

with open('list.txt', 'r') as f:
    text = f.read()
    for k in d:
        before = f'.{k}(?!=\w)'
        print(before)
        after = f'.{d[k]}'
        print(after)
        text = re.sub(before, after, text)

with open('new_list.txt', 'w') as w:
    w.write(text)

you are replacing deliver from the word delivered with xxx. The result is xxxed. Add "delivered": "xxx" to your dictionary. — spyralab
– spyralab, Commented Sep 3, 2020 at 11:43
1) Use word boundaries to match whole words, 2) Escape . outside a character class to match a literal . — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 3, 2020 at 11:46
@spyralab I only want to remove the word after the dot if the key has a value 'deliver' and not 'delivered'. I expect the program to not change anything if the exact match is not found, in this case then it should give the new line as orders.cancelled_at, orders._delivered — trillion
– trillion, Commented Sep 3, 2020 at 11:50
f'\b{k}\b' - this should work ? @WiktorStribiżew sorry I am not that familiar with regex and would appreciate if you can explain more — trillion
– trillion, Commented Sep 3, 2020 at 11:54
\b word boundary is necessary to only match if we have a whole word in the string, so short\b will match in short. and not in shorts. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 7, 2020 at 12:06

Wiktor Stribiżew · Accepted Answer · 2020-09-03 13:24:21Z

1

You can use

import re

d = {'cancelled':'cancelled_at', 'deliver':'xxx'}
rx = re.compile(fr"(?<=\.)(?:{'|'.join(d)})\b")

with open('list.txt', 'r') as f:
    print( re.sub(rx, lambda x: d[x.group()], f.read()) )

See the Python demo

The regex generated by the code looks like

(?<=\.)(?:cancelled|deliver)\b

See the regex demo. Details:

(?<=\.) - a positive lookbehind that matches a location immediately preceded with a literal .
(?:cancelled|deliver) - two alternatives: cancelled or deliver
\b - as whole words, \b is a word boundary.

The lambda x: d[x.group()] replacement replaces the matched word with the corresponding dictionary key value.

answered Sep 3, 2020 at 13:24

Wiktor Stribiżew

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2 Comments

trillion Over a year ago

Hey can you explain when do we use 'f' 'fr' , I can see that you have used fr in the re.complie. Would really appreciate if you can explain the difference between those

Wiktor Stribiżew Over a year ago

@HamzaShehzad r is the raw string literal prefix used to define a string literal where the backslash is not used to form string escape sequences (please read the BONUS top sections in Regular expression works on regex101.com, but not on prod) thread. f is an f-string prefix allowing to use variable interpolation (or variable expansion), i.e. use {varname} inside the string literal to actually concatenate strings you add manually with variables (instead of using str.format)

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