Perl Regex "Not" (negative lookahead)

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21

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

2012-04-04 00:12
by GoldenNewby
what about /jimbob/? what about /bob/apples - ysth 2012-04-04 02:58
For my purposes /jimbob/ was okay, but /bob/apples was no - GoldenNewby 2012-04-04 22:58


21

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

/^\/(?!bob\/)/
2012-04-04 00:23
by anonymous coward
Probably /^\/(?!bob\/)/ , though-- but you got the gist of it. Thanks - GoldenNewby 2012-04-04 00:25
@GoldenNewby Woops, overlooked that part of the questio - anonymous coward 2012-04-04 00:25
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 2012-04-04 03:41
@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 2012-04-04 07:45
@ikegami: $ should be \z more often than not anyway; I am not without appreciation for a reason to use the parallel \Aysth 2012-04-05 05:05
Or, much more readable: m{^/(?!bob/)}mscha 2012-04-17 17:18