Is there any workaround for lookbehind(negative lookahead) in re2j?

[Arushi Rai]

I want to add negative lookahead in my regex and so far I have not found any solution? 

(?!) is not supported in re2j  

here's an example of what i am trying to do,

(?!bar|foo|\d)\b(http:\/\/google.com)

so basically I want http://google.com to not start with bar, foo or any digit

[Alan Donovan]

By design, RE2 doesn't support lookahead, positive or negative, since it would requires an algorithm with worst-case exponential running time.  RE2's algorithm runs in time proportional to the length of the input string.

You can find out more by starting here: https://swtch.com/~rsc/regexp/regexp3.html

[Arushi Rai]

Thank you for your response. 

I understand this. However, I wanted to know if there is any workaround to achieve what I explained in the example? 

[Alan Donovan]

On 2 December 2016 at 15:12, Arushi Rai <arushi...@gmail.com> wrote:

Thank you for your response. 

I understand this. However, I wanted to know if there is any workaround to achieve what I explained in the example? 

You can't do it in a single regexp.  You could use two matchers, one looking for just the target string ("http://google.com") and one looking for the target string preceded by the unwanted stuff.  Then, use program logic to filter out the matches of the first that are also matches of the second.  (The first matcher is so simple you needn't use a regexp.)

[Ivan Blanco]

Hello there, I have made it using two regexs within AND: one contains the things you are looking, and the other has another regex looking for the things you do not want to be after you were looking before. I've used the NOT in the second REGEX:
For example:
=AND(REGEXMATCH(B2;"\bAs\b");NOT(REGEXMATCH(B2;"\bAs (if|success|failure|long|such|\w+ as)\b")))
It looks for any text containing As that is not followed by  if or success or failure or long or such or any word plus as. 

I think this way is much better than normal negative lookahead.

El dia divendres, 2 de desembre de 2016 a les 21:21:16 UTC+1, adonovan va escriure: