Myujth

my input is following

1 blah blah blah @username_. sblah sblah sblah

the output I need is following
username_.

for now, I make this expression

^.*@([a-zA-Z0-9._]+)$

which working in following

1 blah blah blah @username_.

but if I use it for the full line it's not working

so its get the user and delete before the user
but how I can make it delete the rest once it gets the user

Note I use regex101 for testing if you have a better tool please write it below.

Your pattern uses ^$ which means it needs a full match, your pattern is only partial.

By adding a .* it becomes a full regex and it matches as expected.

"/^.*@([a-zA-Z0-9._]+).*$/"

https://3v4l.org/i4pVd

Another way to do it is to use a partial regex like this.

It skips anything up to the @ and then captures all to a dot

$str = "1 blah blah blah @username_. sblah sblah sblah";
preg_match("/.*?@(.*?.)/", $str, $match);

var_dump($match);

https://3v4l.org/mvBYI

To match the username in you example data, you could preg_match and omit the $ to assert the position at the end of the string as in this demo. Note that you don't have to escape the @ and the dot and underscore in the character class.

To get the username in you example data, you could also use:

@K[w.]+

That would match

• 
  @ Match literally


• 
  K Forget what was previously matched


• 
  [w.]+ Match 1+ times a word character or a dot

Regex demo

$re = '/@K[w.]+/';
$str = '1 blah blah blah @username_. sblah sblah sblah @test';
preg_match($re, $str, $matches);
echo $matches[0]; // username_.

Demo php