Negative regex for Perl string pattern match

Question

I have this regex   if  string    m   Clinton   Bush  Reagan  i     print   string n      I want to match with Clinton and Reagan  but not Bush   It s not working

User · Answer

If my understanding is correct then you want to match any line which has Clinton and Reagan, in any order, but not Bush. As suggested by Stuck, here is a version with lookahead assertions:

#!/usr/bin/perl

use strict;
use warnings;

my $regex = qr/
    (?=.*clinton)  
    (?!.*bush) 
    .*reagan       
    /ix;

while (<DATA>) {
    chomp;
    next unless (/$regex/);
    print $_, "\n";
}


__DATA__
shouldn't match - reagan came first, then clinton, finally bush
first match - first two: reagan and clinton
second match - first two reverse: clinton and reagan
shouldn't match - last two: clinton and bush
shouldn't match - reverse: bush and clinton
shouldn't match - and then came obama, along comes mary
shouldn't match - to clinton with perl

Results

first match - first two: reagan and clinton
second match - first two reverse: clinton and reagan

as desired it matches any line which has Reagan and Clinton in any order.

You may want to try reading how lookahead assertions work with examples at http://www252.pair.com/comdog/mastering_perl/Chapters/02.advanced_regular_expressions.html

they are very tasty :)

User · Answer

What s wrong with using two regexs  or three   This makes your intentions more clear and may even improve your performance   if   string       Clinton Reagan  i  amp  amp   string     Bush i           if    string      Clinton i     string      Reagan i           amp  amp   string     Bush i        print   string n

User · Answer

Your regex says the following              - if the line starts with            - start a capture group Clinton    -  Clinton              - or   Bush     - Any single character except  B    u    s  or  h             - or Reagan    -  Reagan   End capture group   i         - Make matches case-insensitive    So  in other words  your middle part of the regex is screwing you up  As it is a  catch-all  kind of group  it will allow any line that does not begin with any of the upper or lower case letters in  Bush   For example  these lines would match your regex   Our president  George Bush In the news today  pigs can fly 012-3123 33   You either make a negative look-ahead  as suggested earlier  or you simply make two regexes   if    string    m   Clinton Reagan  i  and       string    m  Bush i         print   string n       As mirod has pointed out in the comments  the second check is quite unnecessary when using the caret     to match only beginning of lines  as lines that begin with  Clinton  or  Reagan  could never begin with  Bush    However  it would be valid without the carets

User · Answer

Sample text      Clinton said   Bush used crayons   Reagan forgot   Just omitting a Bush match     perl -ne  print if    Clinton Reagan    textfile Clinton said Reagan forgot   Or if you really want to specify     perl -ne  print if      Bush  Clinton Reagan    textfile Clinton said Reagan forgot

User · Answer

Your regex does not work because    defines a character class  but what you want is a lookahead        - Positive look ahead assertion foo   bar  matches foo when followed by bar      - Negative look ahead assertion foo   bar  matches foo when not followed by bar    lt    - Positive look behind assertion    lt  foo bar matches bar when preceded by foo    lt    - Negative look behind assertion    lt  foo bar matches bar when NOT preceded by foo    gt   - Once-only subpatterns    gt  d  bar Performance enhancing when bar not present    x   - Conditional subpatterns    3 foo fu bar - Matches foo if 3rd subpattern has matched  fu if not      - Comment     Pattern does x y or z    So try     bush

[regex] Negative regex for Perl string pattern match

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