Regular Expression: not containing and look-ahead

Question

With this expression: .*(?=good) i can get:

text is good -> text is

But how to supplement this expression so that it also work in the following case:

text is not good -> NOT MATCH or empty string

Answer 1

You could use a negative Lookbehind:

.+(?<!not )(?=good)

This ensures that the position where the Lookahead starts is not preceded by "not ".

Demo.

If you want to make sure that the word "not" doesn't appear anywhere before the Lookahead, you may use:

^(?:(?!not).)+(?=good)

Demo.

And if you want to make sure that the word "not" doesn't appear anywhere in the string (even after the Lookahead), you may use:

^(?!.*not).+(?=good)

Demo.

P.S. Use \bnot\b if the rejected "not" must be a whole word (e.g., if you want to allow "knot").

Answer 2

You can assert from the start of the string that it does not contain not good

Then use a non greedy quantifier to match until the first occurrence of good to the right (In case good occurs more than once)

^(?!.*\bnot good\b).*?(?= good\b)

Explanation

• ^ Start of string
• (?! Negative lookahead
  • .*\bnot good\b Match not good between word boundaries
• ) Close lookahead
• .*? Match any char as least as possible (In case of more occurrences of good, stop before the first)
• (?= good\b) Positive lookahead, assert good at the right

Regex demo