Starting and ending regular expression

I want to issue an alert when only a dot in a line.
It needs check for starting and ending regular expression.

Is it a character string starting with 〇〇？

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preg_match("/^xx/", $char);

Is it a character string ending with 〇〇？

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preg_match("/xx$/", $char);

Here is the main theme. When judging whether it is a character string of only dots, combine the above.

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$char = '.';
 
if(preg_match('/^.$/', $char)){
    echo "please write again.";
} else {
    echo "OK";
}

-> please write again.

Yes, it is as expected.
I would like to apply next. Read the file and if there is a line with only dots, it will issue an error.

this is a popular Hamlet.
test.txt

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To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them? To die: to sleep;
No more; and by a sleep to say we end
The heart-ache and the thousand natural shocks
That flesh is heir to, 'tis a consummation
Devoutly to be wish'd. To die, to sleep;

app.php

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$file = fopen("text.txt", "r");
 
if($file){
    while($line = fgets($file)){
 
        if(preg_match('/^.$/', $char)){
            echo "please write again.<br>";
        } else {
            echo "OK<br>";
        }
 
    }
}


It is looks like it is going well😂