Regular expression to expand to sentence - regex

Question

I'm trying to extract regions around keywords from longer passages of text. They should include complete sentences, based on the following conditions:
n=250 Charactars before / after keyword should be included if existing (the keyword can be closer then this to the start / end of the text)
from there it should expand further to include the complete sentence (let's assume here we can define sentence borders with ".?! or :" knowing it's not completely accurate)
I already achieved the expanding to the end of the last sentence, but not to start of the first in the following example, where vitamin is the keyword and the italic is captured by the regex. However, it should capture from "An extra 24 hours..."
Apparently I don't get the corresponding group up front, neither using lazy nor using lookbehind.
((.{0,250}(vitamin)\b.{0,250})(.+?(\.|\!|\?|\:))?)/ig
Well, this year you’re getting an extra day to get ahead on your taxes or (finally) clean out the garage. (Hey, we’re not trying to tell you what do but you might as well be productive.) February 29 is back on the calendar this year because it’s a leap year. Whether you love or loathe the extra winter day, you’re probably wondering why it happens in the first place. An extra 24 hours — or day — is built into the calen dar every four years to ensure it aligns with the Earth’s movement around the sun. There’s 365 days in a calendar year, but it actually takes longer for the Earth’s annual journey — about 365.2421 days — around the star that gives us light, life and vitamin D. The difference may seem like no big deal to us, but over time, it adds up. “To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in sync with the heavens,” according the history. com.
Acknowledgement of the need for a leap year happened around the time of Julius Caesar. In 46 B.C., Caesar enlisted the help of astronomer Sosigenes to update the calendar so that it had 12 months and 365 days, including a leap year every four years.,

Answer 1

You can try something like this:
(([.?!:][^.?!:]*.{250}\bvitamin\b.{250})[^.?!:]*[.?!:])
It works by consuming 250 characters of text before and after the keyword "vitamin". From that point it finds the first punctuation point (.?!:) before/after the 250 characters of text.
Here's a sample of it in action.
You can you use extra parentheses () to strategically group what exact output you want. For example, the above answer includes the ending period from the preceding sentence in the output. So you could use
(([.?!:]([^.?!:]*.{250}\bvitamin\b.{250})[^.?!:]*[.?!:]))
and use group 3 from the result set which doesn't have this ending period.

Answer 2

I do not see how the specification in the question can be matched by a regex. It boils down to the following logic problem:
to match as many characters as possible but no more than 250 before/after the keyword, .{0,250} needs to be greedy and can neither be lazy .{0,250}? nor possessive .{0,250}+
if this part is greedy, you will miss the occurrences of the keyword that start before the .{0,250} part is matched.
The same logic applies to my understanding to the 'match back to the start of the sentnence as well.
I played around with the following more or less meaningful regex:
[.?!:]?([^.?!:]*?(.{0,250}\byear\b.{0,250})[^.?!:]*[.?!:]?) misses first 'year'
[.?!:]?([^.?!:]*?(.{0,250}?\byear\b.{0,250})[^.?!:]*[.?!:]?) gets the first 'year' but fails on others.
I suggest you write your on extraction logic in a function, eihter using regex or not, to achieve the extraction you want.
You could for example find the index of the start of the keyword \bkeyword\b and the full stops (\.[^\d]|[.?!:]$) and then with this information extract the part of the text you want.