Regular expression - matching word only once per line

Question

Happening:

• ehello goodbye hello hello goodbye
• ehello goodbye hello hello goodbye

I want to match line 1 (only has “hello” once!) DO NOT want to match line 2 (contains “hello” more than once)

I tried to use a negative look ahead and what not ... without real success.

Answer 1

A simple option is this (using a multi-line flag, not a period):

^(?!.*\bhello\b.*\bhello\b).*\bhello\b.*$ 

First, make sure you don't have “hi” twice, and then check that you have at least once. There are other ways to test the same thing, but I think it's pretty simple.

Of course, you can just match for \bhello\b and count the number of matches ...

Answer 2

A common regex would be:

 ^(?:\b(\w+)\b\W*(?!.*?\b\1\b))*\z 

Altho could be cleaner to invert the result of this match:

 \b(\w+)\b(?=.*?\b\1\b) 

This works by matching the word and capturing it, and then checking with lookahead and backreference what it does / doesn't follow anywhere in the line.

Answer 3

Since you are only worried about words (for example, tokens separated by spaces), you can simply break the spaces and see how often "hello" appears. Since you did not specify a language, the implementation in Perl is implemented here:

 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"; 

Output signal

 1, 2