Python regular expression issue

Question

I'm trying to use the re module in a way that it will return bunch of characters until a particular string follows an individual character. The re documentation seems to indicate that I can use (?!...) to accomplish this. The example that I'm currently wrestling with:

str_to_search = 'abababsonab, etc'
first = re.search(r'(ab)+(?!son)', str_to_search)
second = re.search(r'.+(?!son)', str_to_search)

first.group() is 'abab', which is what I'm aiming for. However, second.group() returns the entire str_to_search string, despite the fact that I'm trying to make it stop at 'ababa', as the subsequent 'b' is immediately followed by 'son'. Where am I going wrong?

Are you trying to INCLUDE the full ababab prior to son or just the first abab stopping before abson? — the wolf
– the wolf, Commented Nov 7, 2013 at 19:32

sdanzig · Accepted Answer · 2013-11-07 19:43:41Z

2

It's not the simplest thing, but you can capture a repeating sequence of "a character not followed by 'son'". This repeated expression should be in a non-capturing group, (?: ... ), so it doesn't mess with your match results. (You'd end up with an extra match group)

Try this:

import re

str_to_search = 'abababsonab, etc'
second = re.search(r'(?:.(?!son))+', str_to_search)
print(second.group())

Output:

ababa

See it here: http://ideone.com/6DhLgN

edited Nov 7, 2013 at 19:43

answered Nov 7, 2013 at 19:35

sdanzig

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Comments

score 1 · Accepted Answer · 2013-11-07 19:48:15Z

1

This should work:

second = re.search(r'(.(?!son))+', str_to_search)
#output: 'ababa'

edited Nov 7, 2013 at 19:48

answered Nov 7, 2013 at 19:39

user1301404

3 Comments

sdanzig Over a year ago

Not sure why you used .? and not .

Fred the Fantastic Over a year ago

This worked (and was the easiest for me to wrap my brain around). Thanks for the help everyone!

sdanzig Over a year ago

What @void has is a repeated capture group, which appears as match group 1, and overwrites itself each iteration of the +. That's why I added the ?: ... Just a warning to anyone who tries to understand what's going on exactly with this.

vish · Accepted Answer · 2013-11-07 19:32:16Z

0

not sure what you are trying to do

check out string.partition
'.+?' is the minimal matcher, otherwise it is greedy and gets it all
read the docs for group(...) and groups(..) especially when passing group number

answered Nov 7, 2013 at 19:32

vish

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