How to exclude specific string using regex in Python?

Question

I'd like to match strings like:

45 meters?
45, meters?
45?
45 ?

but not strings like:

45 meters you?
45 you  ?
45, and you?

In both cases the question mark must be at the end. So, essentially I want to exclude all those strings containing the word "you".

I've tried the following regex:

'\d+.*(?!you)\?$'

but it matches the second case (probably because of .*)

It could be anywhere, but for now I'm interested when it occurs at the end, thank you. — f_ficarola, Jun 19 '14 at 16:37
I meant "anywhere" in the sense that between `you` and `?` there could be some white spaces. — f_ficarola, Jun 19 '14 at 16:41
Is regex mandatory? you can make it like: `"you" in mystring` — Ruben Bermudez, Jun 19 '14 at 16:42
The simplest solution is to do 2 passes. On the first pass, keep all of the lines which match a certain expression, on the second pass, filter out all the ones that contain "you". — Joel Cornett, Jun 19 '14 at 16:45

Avinash Raj · Accepted Answer · 2014-06-19T17:57:18.350

11

You could try this regex to match all the lines which doesn't have the string you with ? at the last,

^(?!.*you).*\?$

Explanation:

A negative lookahead is used in this regex. What it does actually means, it checks for the lines which contains a string you. It matches all the lines except the line containing the string you.

DEMO

edited Jun 19 '14 at 17:57

answered Jun 19 '14 at 16:42

Avinash Raj

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The OP only wants it to discard the match if the word is `you`. – Amal Murali Jun 19 '14 at 16:42
1

Sorry, but I need `.`, because it should even match `45, meters?` – f_ficarola Jun 19 '14 at 16:45
did you want this `^[\d,]+ ?(\w+)?\?$`? – Avinash Raj Jun 19 '14 at 16:48
Better, but again, as @AmalMurali said, it fails `42 test meters?` – f_ficarola Jun 19 '14 at 16:52
Yes, I think the second one is the best one, but I'll change it in `^(?!.*you).*\?$'` because as I wrote, I also need the question mark at the end of the string. Thank you!!! – f_ficarola Jun 19 '14 at 17:01
Again, I've changed it to: `^\d+(?!.*you).*\?$` because I need a number at the beginning of the string! :) – f_ficarola Jun 19 '14 at 17:05

Zero Piraeus · Answer 2 · 2014-06-19T16:57:11.583

There's a neat trick to exclude some matches from a regex, which you can use here:

>>> import re
>>> corpus = """
... 45 meters?
... 45?
... 45 ?
... 45 meters you?
... 45 you  ?
... 45, and you?
... """
>>> pattern = re.compile(r"\d+[^?]*you|(\d+[^?]*\?)")
>>> re.findall(pattern, corpus)
['45 meters?', '45?', '45 ?', '', '', '']

The downside is that you get empty matches when the exclusion kicks in, but those are easily filtered out:

>>> filter(None, re.findall(pattern, corpus))
['45 meters?', '45?', '45 ?']

How it works:

The trick is that we only pay attention to captured groups ... so the left hand side of the alternation - \d+[^?]*you (or "digits followed by non-?-characters followed by 'you'") matches what you don't want, and then we forget about it. Only if the left hand side doesn't match is the right hand side - (\d+[^?]*\?) (or "digits followed by non-?-characters followed by '?') - matched, and that one is captured.

+1. I was writing an answer with the same method, but you were faster. — Amal Murali, Jun 19 '14 at 16:58

How to exclude specific string using regex in Python?

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