Regex match word not immediately preceded by another word but possibly preceded by that word before

Question

I need to match all strings that contain one word of a list, but only if that word is not immediately preceded by another specific word. I have this regex:

.*(?<!forbidden)\b(word1|word2|word3)\b.*

that is still matching a sentence like hello forbidden word1 because forbidden is matched by .*. But if I remove the .* I am not anymore matching strings like hello word1, which I want to match.

Note that I want to match a string like forbidden hello word1.

Could you suggest me how to fix this problem?

just get rid of anchors, they serve no purpose here anyways – gog Jun 24 '22 at 09:23 — gog, Jun 24 '22 at 09:23
@gog you are right, but this does not solve the problem – Vitto Jun 24 '22 at 09:26 — Vitto, Jun 24 '22 at 09:26

score 1 · Answer 1 · answered Jun 24 '22 at 09:45

1

This one seems to work well :

^.*\b(?!(?:forbidden|word[1-3])\b)\w+ (word[1-3]).*$

\b(?!(?:forbidden|word[1-3])\b)\w+ checks for multiple following words that are not forbidden or word[1-3].

So it matches hi forbidden hello word1 test but not hi hello forbidden word2 test.

answered Jun 24 '22 at 09:45

razian

11
2

Have you try this: "hi forbidden hello word1 test forbidden word1" – Chicky Jun 24 '22 at 09:50
Oh you're right, `^.*` and `.*$` are incorrect so it's not working with entire strings. – razian Jun 24 '22 at 09:57

Chicky · Answer 2 · 2022-06-24T10:32:09.540

If what you want is match entire string. Try this:

Regex test

The knowledge is from this thread Regular expression to match a line that doesn't contain a word

I've just reversed the order of look-around

^(.(?<!forbidden (word1|word2|word3)\b))* to discard any string that has pattern forbidden (word1|word2|word3)

((?<!forbidden )\b(word1|word2|word3)\b) is what you defined

But I just can't understand why do you need this requirement.

bobble bubble · Accepted Answer · 2022-06-24T13:18:15.693

Have a look into word boundaries \bword can never touch a word character to the left.

To disallow (word1|word2|word3) if not preceded by forbidden and

one \W (non word character)
```
^.*?\b(?<!forbidden\W)(word1|word2|word3)\b.*
```
See this demo at regex101
multiple \W

Lookbehinds need to be of fixed length in Python regex. To get around this, an idea is to use \W* outside preceded by (?<!\W) for setting the position to look behind.
```
^.*?(?<!forbidden)(?<!\W)\W*\b(word1|word2|word3)\b.*
```
Regex101 demo (in multiline demo I used [^\w\n] instead \W for not skipping over lines)

Certainly variable-width lookbehind, such as (?<!forbidden\W+) would be more comfortable. PyPI Regex > import regex AS re supports lookbehind of variable length: See this demo

Note: If you do not capture anything, a (?: non-capturing groups can be used as well.

Regex match word not immediately preceded by another word but possibly preceded by that word before

3 Answers3