regular expression - match word only once in line

Question

Case:

1. ehello goodbye hellot hello goodbye
2. ehello goodbye hello hello goodbye

I want to match line 1 (only has \'hello\' once!) DO NOT w

Answer 1

Since you're only worried about words (ie tokens separated by whitespace), you can just split on spaces and see how often "hello" appears. Since you didn't mention a language, here's an implementation in Perl:

use strict;
use warnings;

my $a1="ehello goodbye hellot hello goodbye";
my $a2="ehello goodbye hello hello goodbye";

my @arr1=split(/\s+/,$a1);
my @arr2=split(/\s+/,$a2);

#grab the number of times that "hello" appears

my $num_hello1=scalar(grep{$_ eq "hello"}@arr1);
my $num_hello2=scalar(grep{$_ eq "hello"}@arr2);

print "$num_hello1, $num_hello2\n";

The output is

1, 2