Negative lookbehind/ahead assertions in Linux less pager (or vim)

Question

I want to find all instances of "index" not followed by .php in a log using less. /index(?!\.php) does not work. Is this possible? What is the regex for less and vim (do they differ?). Is this not possible with these application's respective regex libraries?

For the why part, see [Why does my regular expression work in X but not in Y?](http://unix.stackexchange.com/questions/119905/why-does-my-regular-expression-work-in-x-but-not-in-y) — Gilles 'SO- stop being evil', May 04 '14 at 21:05

cuonglm · Accepted Answer · 2015-03-17T12:43:10.010

22

In vim, you can do like this:

/index\(\.php\)\@!

For more details, in command mode, try :h \@:

\@!     Matches with zero width if the preceding atom does NOT match at the
        current position. /zero-width {not in Vi}
        Like '(?!pattern)" in Perl.
        Example                 matches
        foo\(bar\)\@!           any "foo" not followed by "bar"
        a.\{-}p\@!              "a", "ap", "aap", "app", etc. not immediately
                                followed by a "p"
        if \(\(then\)\@!.\)*$   "if " not followed by "then"

edited Mar 17 '15 at 12:43

answered May 04 '14 at 16:53

cuonglm

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Beautiful! Any idea for less? This doesn't work in less. I wish regex behavior was PCRE everywhere, but alas it isn't. – Gregg Leventhal May 04 '14 at 16:57
7

Also note the syntax for **negativ lookbehind**: `\@ – lanoxx Mar 17 '15 at 12:33
It goes with saying that you need to put the negative look*behind* before the pattern. An example: `$some$\@<!thing` Will match `thing` and `everything` and `nothing`, but not `something`. – dwanderson Mar 07 '17 at 21:26

score 8 · Answer 2 · edited Feb 22 '16 at 13:37

8

(?!\.php) is a perl regexp operator. less generally uses the system's POSIX regexp API, so typically GNU extended regular expressions on a GNU system, vim uses vim regular expressions.

In vim, as already shown by cuonglm, the equivalent of index(?!\.php) would be index$\.php$\@! or \vindex(\.php)@!.

For less, at compile time, you can choose the regex library/API and as a result the regex type to use:

    --with-regex={auto,gnu,pcre,posix,regcmp,re_comp,
                    regcomp,regcomp-local,none}
        Select a regular expression library  auto

By default though, less will use POSIX regcomp with REG_EXTENDED, so you'll get the extended regular expressions of your system, so typically something similar as with grep -E.

In GNU extended regexp, there's no equivalent look behind or look ahead operator.

You could do it the hard way:

index($|[^.]|\.($|([^p]|p($|([^h]|h($|[^p]))))))

With less, you could possibly use the & key to filter out the lines containing index.php (&!index\.php), and then search for index (/index). (you'd still miss the other instances of index that appear on a line also containing index.php).

edited Feb 22 '16 at 13:37

cuonglm

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answered May 04 '14 at 17:20

Stéphane Chazelas

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I think what regex library which `less` uses is depended on compiled time. – cuonglm May 04 '14 at 18:32
@Gnouc, you're right, it even now supports PCRE it seems. – Stéphane Chazelas May 05 '14 at 06:04
Yeah, we can check if `less` uses `PCRE` by parsing output of `ldd $(which less)`. But with other library, do you know any way to check? – cuonglm May 05 '14 at 06:11
1

@Gnouc, it prints the name of the regex library with `less --version`. – Stéphane Chazelas May 05 '14 at 06:28
I use Ubuntu 12.04 LTS and with `less --verion`, it only prints `less 444` along with Copyright. – cuonglm May 05 '14 at 06:45
@Gnouc, yes, it looks like it's only included when explicitly specified at compile time. So if you see nothing, that means less was built with the default. – Stéphane Chazelas May 05 '14 at 07:52

Negative lookbehind/ahead assertions in Linux less pager (or vim)

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