Regex to match a string which does not contain a specific word next to the match string

Question

I have Text as below

 1. This is                678         897        999
    not a text which I want


   2. This is                 678         897        879

I have applied regex as

This\s*is\s*(\s+\d+){1,}(?: ){0,}[\r\n]+

Now what I want is match a string which does not have not next to the match string. I don't want the regex to match first string.

EDIT

Suppose I have 2 string as above and I applied regex then I found 2 match

This is                678         897        999
This is                 678         897        879

Upto this all is perfect but now I want regex which does not contain not(in first string) , I want to match only 2nd string.

Answer 1

This\s*is\s*(\s+\d+){1,}(?: ){0,}(?:[\r\n]+|$)(?!not)

Just add lookahead .See demo.

https://regex101.com/r/eB8xU8/8

Answer 2

I want regex which does not contain not(in first string), I want to match only 2nd string.

That means you should check if the This is... pattern is not followed by newline sequence + spaces* + not as a whole word with backtracking disabled . We can disable backtracking using atomic group in .NET:

(?>This\s+is(?:\s+\d+)+ *)(?![\r\n]+\p{Zs}*not\b)

See the regex demo

Part 1 of the regex This\\s+is(?:\\s+\\d+)+ * matches This is followed with one or more sequences of one or more whitespaces followed with one or more digits, then followed with zero or more spaces. The (?>...) prevent backtracking inside this part of the pattern. The lookahead (?![\\r\\n]+\\p{Zs}*not\\b) fails the match if the previously matched text is followed with the whitespaces followed with a whole word not (where \\b stands for a word boundary).