PHP regex check if contains a specific word but doesn't contain another

Question

How can I check using PHP regular expressions, whether my string variable $str contains the word 'cat' but does not contain the word 'dog'.

• Case 1: $str = "My pet: parrot" -> Output: false
• Case 2: $str = "My pet: dog and cat" -> Output: false
• Case 3: $str = "My pet: cat" -> Output: true

I have tried this and it works but I was wondering if there was a single regular expression to do it

$str = "My pet: dog and cat";
if (preg_match('/(\bdog\b)/', $str) && !preg_match('/(\bcat\b)/', $str)) echo 'TRUE';
else echo 'FALSE';

Answer 1

You can use a negative and a positive lookahead for this:

^(?=.*?cat)(?!.*?dog).*$

To check for complete words use \b:

^(?=.*?\bcat\b)(?!.*?\bdog\b).*$

RegEx Demo