Question

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’

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

Any ideas?

Answer 1

! 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”.

DEMO: http://ideone.com/bOWbu

More info on regex lookarounds: http://www.regular-expressions.info/lookaround.html