Question

How can I write regular expression in which
whenever there is 0000 there should be 1111 after this for example:

00101011000011111111001111010 -> correct
0000110                       -> incorect
11110                         -> correct

thanks for any help

Answer 1

If you are using Perl, you can use a zero-width negative-lookahead assertion:

#!/usr/bin/perl

use strict; use warnings;

my @strings = qw(
    00101011000011111111001111010
    00001111000011
    0000110
    11110
);

my $re = qr/0000(?!1111)/;

for my $s ( @strings ) {
    my $result = $s =~ $re ? 'incorrect' : 'correct';
    print "$s -> $result\n";
}

The pattern matches if there is a string of 0000 not followed by at least four 1s. So, a match indicates an incorrect string.

Output:

C:\Temp> s
00101011000011111111001111010 -> correct
00001111000011 -> incorrect
0000110 -> incorrect
11110 -> correct