Regex matching multiple negative lookaheads

Question

I'm trying to match a string (using a Perl regex) only if it doesn't start with "abc:" or "defg:", but I can't seem to find out how. I've tried something like

^(?:(?!abc:)|(?!defg:))

that asks for any string that doesn't start abc: *or* doesn't start defg:, which is any string — ysth, Nov 27 '14 at 23:47

score 38 · Accepted Answer · edited Apr 28 '23 at 10:12

38

Lookahead (?=foo), (?!foo) and lookbehind (?<=foo), (?<!foo) do not consume any characters.

You can do multiple assertions:

^(?!abc:)(?!defg:)

or:

^(?!defg:)(?!abc:)

...and the order does not make a difference.

You can use De Morgan (as other answers do):

(NOT A) AND (NOT B) <=> NOT (A OR B)

...and shorten the expression to:

^(?!abc:|defg:)

edited Apr 28 '23 at 10:12

Neuron

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answered Nov 14 '17 at 17:44

Stephan Stamm

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score 9 · Answer 2 · edited Dec 06 '21 at 19:51

9

Try doing this:

^(?!(?:abc|defg):)

edited Dec 06 '21 at 19:51

Peter Mortensen

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answered Nov 27 '14 at 23:33

Gilles Quénot

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score 2 · Answer 3 · edited Dec 06 '21 at 19:53

2

Use this regex:

^(?!abc:|defg:)\s*\w+

This will avoid line start with "abc:" and "defg:" as you want.

edited Dec 06 '21 at 19:53

Peter Mortensen

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answered Nov 14 '17 at 08:58

AJP

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score 1 · Answer 4 · edited Dec 06 '21 at 19:52

1

… or we could have dropped the alternation from the original expression:

^(?:(?!abc:)(?!defg:))

edited Dec 06 '21 at 19:52

Peter Mortensen

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answered Nov 28 '14 at 00:55

Abecee

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score 0 · Answer 5 · edited Dec 06 '21 at 19:52

0

^(?:(?!abc:|defg:).)*$

Try this. See the demo at http://regex101.com/r/hQ9xT1/18.

edited Dec 06 '21 at 19:52

Peter Mortensen

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answered Nov 28 '14 at 03:01

vks

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The preceding `\b` are redundant, and OP doesn't say anything about whatever that follows. – nhahtdh Nov 28 '14 at 04:54

score 0 · Answer 6 · edited Dec 06 '21 at 19:52

0

This will do the task:

^(?!(defg|abc):).*

edited Dec 06 '21 at 19:52

Peter Mortensen

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answered Nov 28 '14 at 04:55

blackSmith

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score 0 · Answer 7 · answered Nov 28 '14 at 05:32

Could you please try this:

use strict;
use warnings;
use Cwd;

while(<DATA>)
{
    my $line=$_;
    print $line unless($line=~m/^(abc|defg*)/m);
}

__DATA__
ebc this is testing ebc
dbc this is testing dbc
defg this is testing defg
abc this is testing abc
defg this is testing defg

Regex matching multiple negative lookaheads

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