Prevent matching @mentions if preceded by specific letters

I need to match all mentions without RT before. I've tried adding ! to negate the rt but don't work.

preg_match('/\b!rt\s*@([a-zA-Z0-9_]{1,20})/i', $string, $data);

this must match: hello @user how are you this not: RT @user how are you

I'm not trying to extract usernames or something, what I need is to know if the text has @user without RT.

Solution

! doesn't "negate" in regex. That matches a literal !. What you want is a "Negative Lookbehind".

/(?<!RT)\s@([a-z0-9]{1,20})/i

(?<!RT) means "not preceded by "RT".

This will match the username, the "RT" isn't included as a match.

$match = preg_match('/(?<!RT)\s@([a-z0-9]{1,20})/i', $string);

If $match is 0, it means the string was "RT @user ...". If $match is not 0, that means the string did not start with "RT @user".