Perl Regex "Not" (negative lookahead)

Question

I'm not terribly certain what the correct wording for this type of regex would be, but basically what I'm trying to do is match any string that starts with "/" but is not followed by "bob/", as an example.

So these would match:

/tom/
/tim/
/steve

But these would not

tom
tim
/bob/

I'm sure the answer is terribly simple, but I had a difficult time searching for "regex not" anywhere. I'm sure there is a fancier word for what I want that would pull good results, but I'm not sure what it would be.

Edit: I've changed the title to indicate the correct name for what I was looking for

what about `/jimbob/`? what about `/bob/apples`? – ysth Apr 04 '12 at 02:58 — ysth, Apr 04 '12 at 02:58
For my purposes /jimbob/ was okay, but /bob/apples was not – GoldenNewby Apr 04 '12 at 22:58 — GoldenNewby, Apr 04 '12 at 22:58

score 35 · Accepted Answer · edited Apr 04 '12 at 03:43

35

You can use a negative lookahead (documented under "Extended Patterns" in perlre):

/^\/(?!bob\/)/

edited Apr 04 '12 at 03:43

brian d foy

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answered Apr 04 '12 at 00:23

Probably /^\/(?!bob\/)/ , though-- but you got the gist of it. Thanks! – GoldenNewby Apr 04 '12 at 00:25
@GoldenNewby Woops, overlooked that part of the question – Apr 04 '12 at 00:25
2

With default regex flags now a feature, that `^` is better written as `\A`. With alternate delimiters and `/x`, it looks like `m| \A / (?!bob/) |x`. – brian d foy Apr 04 '12 at 03:41
9

@brian d foy, It's preposterous to suggest the existence this feature means `/^/` has to be changed to `/\A/`, `/$/` to `/(?=\n?\z)/`, `/./` to `/[^\n]/` and `/ /` to `/[ ]/`. – ikegami Apr 04 '12 at 07:45
@ikegami: `$` should be `\z` more often than not anyway; I am not without appreciation for a reason to use the parallel `\A` – ysth Apr 05 '12 at 05:05

score 8 · Answer 2 · answered Jul 05 '21 at 19:25

TLDR: Negative Lookaheads

If you wanted a negative lookahead just to find "foo" when it isn't followed by "bar"...

$string =~ m/foo(?!bar)/g;

Working Demo Online

Source

To quote the docs...

(?!pattern)

(*nla:pattern)

#(*negative_lookahead:pattern)

A zero-width negative lookahead assertion. For example /foo(?!bar)/ matches any occurrence of "foo" that isn't followed by "bar". Note however that lookahead and lookbehind are NOT the same thing. You cannot use this for lookbehind. (Source: PerlDocs.)

Negative Lookaheads For Your Case

The accepted answer is great, but it leaves no explanation, so let me add one...

/^\/(?!bob\/)/

^ — Match only the start of strings.
\/ — Match the / char, which we need to escape because it is a character in the regex format (i.e. s/find/replacewith/, etc.).
(?!...) — Do not match if the match is followed by ....
bob\/ — This is the ... value, don't match bob/', once more, we need to escape the /`.

Perl Regex "Not" (negative lookahead)

2 Answers2

TLDR: Negative Lookaheads

Source

Negative Lookaheads For Your Case