Let's say you have the following Perl script.

#!/usr/bin/perl
use strict;
use warnings;

my $string = "foo bar foo bar foo bar";

print "$string \n";

Running this script should return the following.

foo bar foo bar foo bar

The following regular expression will match the first occurrence of "foo".

$string =~ s|foo||;

Which should return the following.

bar foo bar foo bar

The g (global) flag should match every occurrence of "foo".

$string =~ s|foo||g;

Which should return the following.

bar bar bar

A negative lookahead can be used to match the last occurrence of "foo".

$string =~ s|foo(?!.*foo)||;

Which should return the following.

foo bar foo bar bar

A different approach is needed when you want to match everything until the first occurrence of a pattern is found. The following will match everything until the first occurrence of bar is found.

$string =~ s|^([^bar]+)||;

Which should return the following.

bar foo bar foo bar