Uppercase first sentence in every post

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Welcome to WPSE.

So I can’t write all the code for you, but I would suggest you look at these main pieces, and then come back here or make new questions if you have followup questions:

  1. To alter the content of a post before it’s printed to the screen you could use the the_content filter. This lets you make any changes you want at all to the content section of a post (or page) before it’s printed to the screen, and return it with any changes to wherever is trying to print it.

  2. Then you have to find the part you want and only change that part. This is more of a PHP question, but for example this snippet of code will take a string with ... in it and split it into two pieces around the ...:

$content = "TITLE HERE... Once upon a time";
$pieces = explode("...", $content, 2);
// now $pieces is a PHP array with anything before "..." in $pieces[0]
// and anything after "..." in $pieces[1]
  1. Then you need to convert some of it to upper case, probably using the PHP function strtoupper() and put it back together. E.g. continuing from above:
$new_content = strtoupper($pieces[0]) . "..." . $pieces[1];

Note: It’s not a very clearly drawn line, but Item 1 is the main WordPress part, Items 2 and 3 are more PHP questions which should be directed to e.g. stackoverflow.com.

HTH and happy to help further, to make best use of this StackExchange is best to make a start on some piece of the code and ask specific questions about where you get to.